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Source file src/go/parser/parser.go

Documentation: go/parser

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package parser implements a parser for Go source files. Input may be
     6  // provided in a variety of forms (see the various Parse* functions); the
     7  // output is an abstract syntax tree (AST) representing the Go source. The
     8  // parser is invoked through one of the Parse* functions.
     9  //
    10  // The parser accepts a larger language than is syntactically permitted by
    11  // the Go spec, for simplicity, and for improved robustness in the presence
    12  // of syntax errors. For instance, in method declarations, the receiver is
    13  // treated like an ordinary parameter list and thus may contain multiple
    14  // entries where the spec permits exactly one. Consequently, the corresponding
    15  // field in the AST (ast.FuncDecl.Recv) field is not restricted to one entry.
    16  //
    17  package parser
    18  
    19  import (
    20  	"fmt"
    21  	"go/ast"
    22  	"go/scanner"
    23  	"go/token"
    24  	"strconv"
    25  	"strings"
    26  	"unicode"
    27  )
    28  
    29  // The parser structure holds the parser's internal state.
    30  type parser struct {
    31  	file    *token.File
    32  	errors  scanner.ErrorList
    33  	scanner scanner.Scanner
    34  
    35  	// Tracing/debugging
    36  	mode   Mode // parsing mode
    37  	trace  bool // == (mode & Trace != 0)
    38  	indent int  // indentation used for tracing output
    39  
    40  	// Comments
    41  	comments    []*ast.CommentGroup
    42  	leadComment *ast.CommentGroup // last lead comment
    43  	lineComment *ast.CommentGroup // last line comment
    44  
    45  	// Next token
    46  	pos token.Pos   // token position
    47  	tok token.Token // one token look-ahead
    48  	lit string      // token literal
    49  
    50  	// Error recovery
    51  	// (used to limit the number of calls to parser.advance
    52  	// w/o making scanning progress - avoids potential endless
    53  	// loops across multiple parser functions during error recovery)
    54  	syncPos token.Pos // last synchronization position
    55  	syncCnt int       // number of parser.advance calls without progress
    56  
    57  	// Non-syntactic parser control
    58  	exprLev int  // < 0: in control clause, >= 0: in expression
    59  	inRhs   bool // if set, the parser is parsing a rhs expression
    60  
    61  	// Ordinary identifier scopes
    62  	pkgScope   *ast.Scope        // pkgScope.Outer == nil
    63  	topScope   *ast.Scope        // top-most scope; may be pkgScope
    64  	unresolved []*ast.Ident      // unresolved identifiers
    65  	imports    []*ast.ImportSpec // list of imports
    66  
    67  	// Label scopes
    68  	// (maintained by open/close LabelScope)
    69  	labelScope  *ast.Scope     // label scope for current function
    70  	targetStack [][]*ast.Ident // stack of unresolved labels
    71  }
    72  
    73  func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
    74  	p.file = fset.AddFile(filename, -1, len(src))
    75  	var m scanner.Mode
    76  	if mode&ParseComments != 0 {
    77  		m = scanner.ScanComments
    78  	}
    79  	eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
    80  	p.scanner.Init(p.file, src, eh, m)
    81  
    82  	p.mode = mode
    83  	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
    84  
    85  	p.next()
    86  }
    87  
    88  // ----------------------------------------------------------------------------
    89  // Scoping support
    90  
    91  func (p *parser) openScope() {
    92  	p.topScope = ast.NewScope(p.topScope)
    93  }
    94  
    95  func (p *parser) closeScope() {
    96  	p.topScope = p.topScope.Outer
    97  }
    98  
    99  func (p *parser) openLabelScope() {
   100  	p.labelScope = ast.NewScope(p.labelScope)
   101  	p.targetStack = append(p.targetStack, nil)
   102  }
   103  
   104  func (p *parser) closeLabelScope() {
   105  	// resolve labels
   106  	n := len(p.targetStack) - 1
   107  	scope := p.labelScope
   108  	for _, ident := range p.targetStack[n] {
   109  		ident.Obj = scope.Lookup(ident.Name)
   110  		if ident.Obj == nil && p.mode&DeclarationErrors != 0 {
   111  			p.error(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
   112  		}
   113  	}
   114  	// pop label scope
   115  	p.targetStack = p.targetStack[0:n]
   116  	p.labelScope = p.labelScope.Outer
   117  }
   118  
   119  func (p *parser) declare(decl, data interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
   120  	for _, ident := range idents {
   121  		assert(ident.Obj == nil, "identifier already declared or resolved")
   122  		obj := ast.NewObj(kind, ident.Name)
   123  		// remember the corresponding declaration for redeclaration
   124  		// errors and global variable resolution/typechecking phase
   125  		obj.Decl = decl
   126  		obj.Data = data
   127  		ident.Obj = obj
   128  		if ident.Name != "_" {
   129  			if alt := scope.Insert(obj); alt != nil && p.mode&DeclarationErrors != 0 {
   130  				prevDecl := ""
   131  				if pos := alt.Pos(); pos.IsValid() {
   132  					prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.file.Position(pos))
   133  				}
   134  				p.error(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
   135  			}
   136  		}
   137  	}
   138  }
   139  
   140  func (p *parser) shortVarDecl(decl *ast.AssignStmt, list []ast.Expr) {
   141  	// Go spec: A short variable declaration may redeclare variables
   142  	// provided they were originally declared in the same block with
   143  	// the same type, and at least one of the non-blank variables is new.
   144  	n := 0 // number of new variables
   145  	for _, x := range list {
   146  		if ident, isIdent := x.(*ast.Ident); isIdent {
   147  			assert(ident.Obj == nil, "identifier already declared or resolved")
   148  			obj := ast.NewObj(ast.Var, ident.Name)
   149  			// remember corresponding assignment for other tools
   150  			obj.Decl = decl
   151  			ident.Obj = obj
   152  			if ident.Name != "_" {
   153  				if alt := p.topScope.Insert(obj); alt != nil {
   154  					ident.Obj = alt // redeclaration
   155  				} else {
   156  					n++ // new declaration
   157  				}
   158  			}
   159  		} else {
   160  			p.errorExpected(x.Pos(), "identifier on left side of :=")
   161  		}
   162  	}
   163  	if n == 0 && p.mode&DeclarationErrors != 0 {
   164  		p.error(list[0].Pos(), "no new variables on left side of :=")
   165  	}
   166  }
   167  
   168  // The unresolved object is a sentinel to mark identifiers that have been added
   169  // to the list of unresolved identifiers. The sentinel is only used for verifying
   170  // internal consistency.
   171  var unresolved = new(ast.Object)
   172  
   173  // If x is an identifier, tryResolve attempts to resolve x by looking up
   174  // the object it denotes. If no object is found and collectUnresolved is
   175  // set, x is marked as unresolved and collected in the list of unresolved
   176  // identifiers.
   177  //
   178  func (p *parser) tryResolve(x ast.Expr, collectUnresolved bool) {
   179  	// nothing to do if x is not an identifier or the blank identifier
   180  	ident, _ := x.(*ast.Ident)
   181  	if ident == nil {
   182  		return
   183  	}
   184  	assert(ident.Obj == nil, "identifier already declared or resolved")
   185  	if ident.Name == "_" {
   186  		return
   187  	}
   188  	// try to resolve the identifier
   189  	for s := p.topScope; s != nil; s = s.Outer {
   190  		if obj := s.Lookup(ident.Name); obj != nil {
   191  			ident.Obj = obj
   192  			return
   193  		}
   194  	}
   195  	// all local scopes are known, so any unresolved identifier
   196  	// must be found either in the file scope, package scope
   197  	// (perhaps in another file), or universe scope --- collect
   198  	// them so that they can be resolved later
   199  	if collectUnresolved {
   200  		ident.Obj = unresolved
   201  		p.unresolved = append(p.unresolved, ident)
   202  	}
   203  }
   204  
   205  func (p *parser) resolve(x ast.Expr) {
   206  	p.tryResolve(x, true)
   207  }
   208  
   209  // ----------------------------------------------------------------------------
   210  // Parsing support
   211  
   212  func (p *parser) printTrace(a ...interface{}) {
   213  	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
   214  	const n = len(dots)
   215  	pos := p.file.Position(p.pos)
   216  	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
   217  	i := 2 * p.indent
   218  	for i > n {
   219  		fmt.Print(dots)
   220  		i -= n
   221  	}
   222  	// i <= n
   223  	fmt.Print(dots[0:i])
   224  	fmt.Println(a...)
   225  }
   226  
   227  func trace(p *parser, msg string) *parser {
   228  	p.printTrace(msg, "(")
   229  	p.indent++
   230  	return p
   231  }
   232  
   233  // Usage pattern: defer un(trace(p, "..."))
   234  func un(p *parser) {
   235  	p.indent--
   236  	p.printTrace(")")
   237  }
   238  
   239  // Advance to the next token.
   240  func (p *parser) next0() {
   241  	// Because of one-token look-ahead, print the previous token
   242  	// when tracing as it provides a more readable output. The
   243  	// very first token (!p.pos.IsValid()) is not initialized
   244  	// (it is token.ILLEGAL), so don't print it .
   245  	if p.trace && p.pos.IsValid() {
   246  		s := p.tok.String()
   247  		switch {
   248  		case p.tok.IsLiteral():
   249  			p.printTrace(s, p.lit)
   250  		case p.tok.IsOperator(), p.tok.IsKeyword():
   251  			p.printTrace("\"" + s + "\"")
   252  		default:
   253  			p.printTrace(s)
   254  		}
   255  	}
   256  
   257  	p.pos, p.tok, p.lit = p.scanner.Scan()
   258  }
   259  
   260  // Consume a comment and return it and the line on which it ends.
   261  func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
   262  	// /*-style comments may end on a different line than where they start.
   263  	// Scan the comment for '\n' chars and adjust endline accordingly.
   264  	endline = p.file.Line(p.pos)
   265  	if p.lit[1] == '*' {
   266  		// don't use range here - no need to decode Unicode code points
   267  		for i := 0; i < len(p.lit); i++ {
   268  			if p.lit[i] == '\n' {
   269  				endline++
   270  			}
   271  		}
   272  	}
   273  
   274  	comment = &ast.Comment{Slash: p.pos, Text: p.lit}
   275  	p.next0()
   276  
   277  	return
   278  }
   279  
   280  // Consume a group of adjacent comments, add it to the parser's
   281  // comments list, and return it together with the line at which
   282  // the last comment in the group ends. A non-comment token or n
   283  // empty lines terminate a comment group.
   284  //
   285  func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline int) {
   286  	var list []*ast.Comment
   287  	endline = p.file.Line(p.pos)
   288  	for p.tok == token.COMMENT && p.file.Line(p.pos) <= endline+n {
   289  		var comment *ast.Comment
   290  		comment, endline = p.consumeComment()
   291  		list = append(list, comment)
   292  	}
   293  
   294  	// add comment group to the comments list
   295  	comments = &ast.CommentGroup{List: list}
   296  	p.comments = append(p.comments, comments)
   297  
   298  	return
   299  }
   300  
   301  // Advance to the next non-comment token. In the process, collect
   302  // any comment groups encountered, and remember the last lead and
   303  // and line comments.
   304  //
   305  // A lead comment is a comment group that starts and ends in a
   306  // line without any other tokens and that is followed by a non-comment
   307  // token on the line immediately after the comment group.
   308  //
   309  // A line comment is a comment group that follows a non-comment
   310  // token on the same line, and that has no tokens after it on the line
   311  // where it ends.
   312  //
   313  // Lead and line comments may be considered documentation that is
   314  // stored in the AST.
   315  //
   316  func (p *parser) next() {
   317  	p.leadComment = nil
   318  	p.lineComment = nil
   319  	prev := p.pos
   320  	p.next0()
   321  
   322  	if p.tok == token.COMMENT {
   323  		var comment *ast.CommentGroup
   324  		var endline int
   325  
   326  		if p.file.Line(p.pos) == p.file.Line(prev) {
   327  			// The comment is on same line as the previous token; it
   328  			// cannot be a lead comment but may be a line comment.
   329  			comment, endline = p.consumeCommentGroup(0)
   330  			if p.file.Line(p.pos) != endline || p.tok == token.EOF {
   331  				// The next token is on a different line, thus
   332  				// the last comment group is a line comment.
   333  				p.lineComment = comment
   334  			}
   335  		}
   336  
   337  		// consume successor comments, if any
   338  		endline = -1
   339  		for p.tok == token.COMMENT {
   340  			comment, endline = p.consumeCommentGroup(1)
   341  		}
   342  
   343  		if endline+1 == p.file.Line(p.pos) {
   344  			// The next token is following on the line immediately after the
   345  			// comment group, thus the last comment group is a lead comment.
   346  			p.leadComment = comment
   347  		}
   348  	}
   349  }
   350  
   351  // A bailout panic is raised to indicate early termination.
   352  type bailout struct{}
   353  
   354  func (p *parser) error(pos token.Pos, msg string) {
   355  	epos := p.file.Position(pos)
   356  
   357  	// If AllErrors is not set, discard errors reported on the same line
   358  	// as the last recorded error and stop parsing if there are more than
   359  	// 10 errors.
   360  	if p.mode&AllErrors == 0 {
   361  		n := len(p.errors)
   362  		if n > 0 && p.errors[n-1].Pos.Line == epos.Line {
   363  			return // discard - likely a spurious error
   364  		}
   365  		if n > 10 {
   366  			panic(bailout{})
   367  		}
   368  	}
   369  
   370  	p.errors.Add(epos, msg)
   371  }
   372  
   373  func (p *parser) errorExpected(pos token.Pos, msg string) {
   374  	msg = "expected " + msg
   375  	if pos == p.pos {
   376  		// the error happened at the current position;
   377  		// make the error message more specific
   378  		switch {
   379  		case p.tok == token.SEMICOLON && p.lit == "\n":
   380  			msg += ", found newline"
   381  		case p.tok.IsLiteral():
   382  			// print 123 rather than 'INT', etc.
   383  			msg += ", found " + p.lit
   384  		default:
   385  			msg += ", found '" + p.tok.String() + "'"
   386  		}
   387  	}
   388  	p.error(pos, msg)
   389  }
   390  
   391  func (p *parser) expect(tok token.Token) token.Pos {
   392  	pos := p.pos
   393  	if p.tok != tok {
   394  		p.errorExpected(pos, "'"+tok.String()+"'")
   395  	}
   396  	p.next() // make progress
   397  	return pos
   398  }
   399  
   400  // expectClosing is like expect but provides a better error message
   401  // for the common case of a missing comma before a newline.
   402  //
   403  func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
   404  	if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
   405  		p.error(p.pos, "missing ',' before newline in "+context)
   406  		p.next()
   407  	}
   408  	return p.expect(tok)
   409  }
   410  
   411  func (p *parser) expectSemi() {
   412  	// semicolon is optional before a closing ')' or '}'
   413  	if p.tok != token.RPAREN && p.tok != token.RBRACE {
   414  		switch p.tok {
   415  		case token.COMMA:
   416  			// permit a ',' instead of a ';' but complain
   417  			p.errorExpected(p.pos, "';'")
   418  			fallthrough
   419  		case token.SEMICOLON:
   420  			p.next()
   421  		default:
   422  			p.errorExpected(p.pos, "';'")
   423  			p.advance(stmtStart)
   424  		}
   425  	}
   426  }
   427  
   428  func (p *parser) atComma(context string, follow token.Token) bool {
   429  	if p.tok == token.COMMA {
   430  		return true
   431  	}
   432  	if p.tok != follow {
   433  		msg := "missing ','"
   434  		if p.tok == token.SEMICOLON && p.lit == "\n" {
   435  			msg += " before newline"
   436  		}
   437  		p.error(p.pos, msg+" in "+context)
   438  		return true // "insert" comma and continue
   439  	}
   440  	return false
   441  }
   442  
   443  func assert(cond bool, msg string) {
   444  	if !cond {
   445  		panic("go/parser internal error: " + msg)
   446  	}
   447  }
   448  
   449  // advance consumes tokens until the current token p.tok
   450  // is in the 'to' set, or token.EOF. For error recovery.
   451  func (p *parser) advance(to map[token.Token]bool) {
   452  	for ; p.tok != token.EOF; p.next() {
   453  		if to[p.tok] {
   454  			// Return only if parser made some progress since last
   455  			// sync or if it has not reached 10 advance calls without
   456  			// progress. Otherwise consume at least one token to
   457  			// avoid an endless parser loop (it is possible that
   458  			// both parseOperand and parseStmt call advance and
   459  			// correctly do not advance, thus the need for the
   460  			// invocation limit p.syncCnt).
   461  			if p.pos == p.syncPos && p.syncCnt < 10 {
   462  				p.syncCnt++
   463  				return
   464  			}
   465  			if p.pos > p.syncPos {
   466  				p.syncPos = p.pos
   467  				p.syncCnt = 0
   468  				return
   469  			}
   470  			// Reaching here indicates a parser bug, likely an
   471  			// incorrect token list in this function, but it only
   472  			// leads to skipping of possibly correct code if a
   473  			// previous error is present, and thus is preferred
   474  			// over a non-terminating parse.
   475  		}
   476  	}
   477  }
   478  
   479  var stmtStart = map[token.Token]bool{
   480  	token.BREAK:       true,
   481  	token.CONST:       true,
   482  	token.CONTINUE:    true,
   483  	token.DEFER:       true,
   484  	token.FALLTHROUGH: true,
   485  	token.FOR:         true,
   486  	token.GO:          true,
   487  	token.GOTO:        true,
   488  	token.IF:          true,
   489  	token.RETURN:      true,
   490  	token.SELECT:      true,
   491  	token.SWITCH:      true,
   492  	token.TYPE:        true,
   493  	token.VAR:         true,
   494  }
   495  
   496  var declStart = map[token.Token]bool{
   497  	token.CONST: true,
   498  	token.TYPE:  true,
   499  	token.VAR:   true,
   500  }
   501  
   502  var exprEnd = map[token.Token]bool{
   503  	token.COMMA:     true,
   504  	token.COLON:     true,
   505  	token.SEMICOLON: true,
   506  	token.RPAREN:    true,
   507  	token.RBRACK:    true,
   508  	token.RBRACE:    true,
   509  }
   510  
   511  // safePos returns a valid file position for a given position: If pos
   512  // is valid to begin with, safePos returns pos. If pos is out-of-range,
   513  // safePos returns the EOF position.
   514  //
   515  // This is hack to work around "artificial" end positions in the AST which
   516  // are computed by adding 1 to (presumably valid) token positions. If the
   517  // token positions are invalid due to parse errors, the resulting end position
   518  // may be past the file's EOF position, which would lead to panics if used
   519  // later on.
   520  //
   521  func (p *parser) safePos(pos token.Pos) (res token.Pos) {
   522  	defer func() {
   523  		if recover() != nil {
   524  			res = token.Pos(p.file.Base() + p.file.Size()) // EOF position
   525  		}
   526  	}()
   527  	_ = p.file.Offset(pos) // trigger a panic if position is out-of-range
   528  	return pos
   529  }
   530  
   531  // ----------------------------------------------------------------------------
   532  // Identifiers
   533  
   534  func (p *parser) parseIdent() *ast.Ident {
   535  	pos := p.pos
   536  	name := "_"
   537  	if p.tok == token.IDENT {
   538  		name = p.lit
   539  		p.next()
   540  	} else {
   541  		p.expect(token.IDENT) // use expect() error handling
   542  	}
   543  	return &ast.Ident{NamePos: pos, Name: name}
   544  }
   545  
   546  func (p *parser) parseIdentList() (list []*ast.Ident) {
   547  	if p.trace {
   548  		defer un(trace(p, "IdentList"))
   549  	}
   550  
   551  	list = append(list, p.parseIdent())
   552  	for p.tok == token.COMMA {
   553  		p.next()
   554  		list = append(list, p.parseIdent())
   555  	}
   556  
   557  	return
   558  }
   559  
   560  // ----------------------------------------------------------------------------
   561  // Common productions
   562  
   563  // If lhs is set, result list elements which are identifiers are not resolved.
   564  func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
   565  	if p.trace {
   566  		defer un(trace(p, "ExpressionList"))
   567  	}
   568  
   569  	list = append(list, p.checkExpr(p.parseExpr(lhs)))
   570  	for p.tok == token.COMMA {
   571  		p.next()
   572  		list = append(list, p.checkExpr(p.parseExpr(lhs)))
   573  	}
   574  
   575  	return
   576  }
   577  
   578  func (p *parser) parseLhsList() []ast.Expr {
   579  	old := p.inRhs
   580  	p.inRhs = false
   581  	list := p.parseExprList(true)
   582  	switch p.tok {
   583  	case token.DEFINE:
   584  		// lhs of a short variable declaration
   585  		// but doesn't enter scope until later:
   586  		// caller must call p.shortVarDecl(p.makeIdentList(list))
   587  		// at appropriate time.
   588  	case token.COLON:
   589  		// lhs of a label declaration or a communication clause of a select
   590  		// statement (parseLhsList is not called when parsing the case clause
   591  		// of a switch statement):
   592  		// - labels are declared by the caller of parseLhsList
   593  		// - for communication clauses, if there is a stand-alone identifier
   594  		//   followed by a colon, we have a syntax error; there is no need
   595  		//   to resolve the identifier in that case
   596  	default:
   597  		// identifiers must be declared elsewhere
   598  		for _, x := range list {
   599  			p.resolve(x)
   600  		}
   601  	}
   602  	p.inRhs = old
   603  	return list
   604  }
   605  
   606  func (p *parser) parseRhsList() []ast.Expr {
   607  	old := p.inRhs
   608  	p.inRhs = true
   609  	list := p.parseExprList(false)
   610  	p.inRhs = old
   611  	return list
   612  }
   613  
   614  // ----------------------------------------------------------------------------
   615  // Types
   616  
   617  func (p *parser) parseType() ast.Expr {
   618  	if p.trace {
   619  		defer un(trace(p, "Type"))
   620  	}
   621  
   622  	typ := p.tryType()
   623  
   624  	if typ == nil {
   625  		pos := p.pos
   626  		p.errorExpected(pos, "type")
   627  		p.advance(exprEnd)
   628  		return &ast.BadExpr{From: pos, To: p.pos}
   629  	}
   630  
   631  	return typ
   632  }
   633  
   634  // If the result is an identifier, it is not resolved.
   635  func (p *parser) parseTypeName() ast.Expr {
   636  	if p.trace {
   637  		defer un(trace(p, "TypeName"))
   638  	}
   639  
   640  	ident := p.parseIdent()
   641  	// don't resolve ident yet - it may be a parameter or field name
   642  
   643  	if p.tok == token.PERIOD {
   644  		// ident is a package name
   645  		p.next()
   646  		p.resolve(ident)
   647  		sel := p.parseIdent()
   648  		return &ast.SelectorExpr{X: ident, Sel: sel}
   649  	}
   650  
   651  	return ident
   652  }
   653  
   654  func (p *parser) parseArrayType() ast.Expr {
   655  	if p.trace {
   656  		defer un(trace(p, "ArrayType"))
   657  	}
   658  
   659  	lbrack := p.expect(token.LBRACK)
   660  	p.exprLev++
   661  	var len ast.Expr
   662  	// always permit ellipsis for more fault-tolerant parsing
   663  	if p.tok == token.ELLIPSIS {
   664  		len = &ast.Ellipsis{Ellipsis: p.pos}
   665  		p.next()
   666  	} else if p.tok != token.RBRACK {
   667  		len = p.parseRhs()
   668  	}
   669  	p.exprLev--
   670  	p.expect(token.RBRACK)
   671  	elt := p.parseType()
   672  
   673  	return &ast.ArrayType{Lbrack: lbrack, Len: len, Elt: elt}
   674  }
   675  
   676  func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
   677  	idents := make([]*ast.Ident, len(list))
   678  	for i, x := range list {
   679  		ident, isIdent := x.(*ast.Ident)
   680  		if !isIdent {
   681  			if _, isBad := x.(*ast.BadExpr); !isBad {
   682  				// only report error if it's a new one
   683  				p.errorExpected(x.Pos(), "identifier")
   684  			}
   685  			ident = &ast.Ident{NamePos: x.Pos(), Name: "_"}
   686  		}
   687  		idents[i] = ident
   688  	}
   689  	return idents
   690  }
   691  
   692  func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
   693  	if p.trace {
   694  		defer un(trace(p, "FieldDecl"))
   695  	}
   696  
   697  	doc := p.leadComment
   698  
   699  	// 1st FieldDecl
   700  	// A type name used as an anonymous field looks like a field identifier.
   701  	var list []ast.Expr
   702  	for {
   703  		list = append(list, p.parseVarType(false))
   704  		if p.tok != token.COMMA {
   705  			break
   706  		}
   707  		p.next()
   708  	}
   709  
   710  	typ := p.tryVarType(false)
   711  
   712  	// analyze case
   713  	var idents []*ast.Ident
   714  	if typ != nil {
   715  		// IdentifierList Type
   716  		idents = p.makeIdentList(list)
   717  	} else {
   718  		// ["*"] TypeName (AnonymousField)
   719  		typ = list[0] // we always have at least one element
   720  		if n := len(list); n > 1 {
   721  			p.errorExpected(p.pos, "type")
   722  			typ = &ast.BadExpr{From: p.pos, To: p.pos}
   723  		} else if !isTypeName(deref(typ)) {
   724  			p.errorExpected(typ.Pos(), "anonymous field")
   725  			typ = &ast.BadExpr{From: typ.Pos(), To: p.safePos(typ.End())}
   726  		}
   727  	}
   728  
   729  	// Tag
   730  	var tag *ast.BasicLit
   731  	if p.tok == token.STRING {
   732  		tag = &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
   733  		p.next()
   734  	}
   735  
   736  	p.expectSemi() // call before accessing p.linecomment
   737  
   738  	field := &ast.Field{Doc: doc, Names: idents, Type: typ, Tag: tag, Comment: p.lineComment}
   739  	p.declare(field, nil, scope, ast.Var, idents...)
   740  	p.resolve(typ)
   741  
   742  	return field
   743  }
   744  
   745  func (p *parser) parseStructType() *ast.StructType {
   746  	if p.trace {
   747  		defer un(trace(p, "StructType"))
   748  	}
   749  
   750  	pos := p.expect(token.STRUCT)
   751  	lbrace := p.expect(token.LBRACE)
   752  	scope := ast.NewScope(nil) // struct scope
   753  	var list []*ast.Field
   754  	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
   755  		// a field declaration cannot start with a '(' but we accept
   756  		// it here for more robust parsing and better error messages
   757  		// (parseFieldDecl will check and complain if necessary)
   758  		list = append(list, p.parseFieldDecl(scope))
   759  	}
   760  	rbrace := p.expect(token.RBRACE)
   761  
   762  	return &ast.StructType{
   763  		Struct: pos,
   764  		Fields: &ast.FieldList{
   765  			Opening: lbrace,
   766  			List:    list,
   767  			Closing: rbrace,
   768  		},
   769  	}
   770  }
   771  
   772  func (p *parser) parsePointerType() *ast.StarExpr {
   773  	if p.trace {
   774  		defer un(trace(p, "PointerType"))
   775  	}
   776  
   777  	star := p.expect(token.MUL)
   778  	base := p.parseType()
   779  
   780  	return &ast.StarExpr{Star: star, X: base}
   781  }
   782  
   783  // If the result is an identifier, it is not resolved.
   784  func (p *parser) tryVarType(isParam bool) ast.Expr {
   785  	if isParam && p.tok == token.ELLIPSIS {
   786  		pos := p.pos
   787  		p.next()
   788  		typ := p.tryIdentOrType() // don't use parseType so we can provide better error message
   789  		if typ != nil {
   790  			p.resolve(typ)
   791  		} else {
   792  			p.error(pos, "'...' parameter is missing type")
   793  			typ = &ast.BadExpr{From: pos, To: p.pos}
   794  		}
   795  		return &ast.Ellipsis{Ellipsis: pos, Elt: typ}
   796  	}
   797  	return p.tryIdentOrType()
   798  }
   799  
   800  // If the result is an identifier, it is not resolved.
   801  func (p *parser) parseVarType(isParam bool) ast.Expr {
   802  	typ := p.tryVarType(isParam)
   803  	if typ == nil {
   804  		pos := p.pos
   805  		p.errorExpected(pos, "type")
   806  		p.next() // make progress
   807  		typ = &ast.BadExpr{From: pos, To: p.pos}
   808  	}
   809  	return typ
   810  }
   811  
   812  func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params []*ast.Field) {
   813  	if p.trace {
   814  		defer un(trace(p, "ParameterList"))
   815  	}
   816  
   817  	// 1st ParameterDecl
   818  	// A list of identifiers looks like a list of type names.
   819  	var list []ast.Expr
   820  	for {
   821  		list = append(list, p.parseVarType(ellipsisOk))
   822  		if p.tok != token.COMMA {
   823  			break
   824  		}
   825  		p.next()
   826  		if p.tok == token.RPAREN {
   827  			break
   828  		}
   829  	}
   830  
   831  	// analyze case
   832  	if typ := p.tryVarType(ellipsisOk); typ != nil {
   833  		// IdentifierList Type
   834  		idents := p.makeIdentList(list)
   835  		field := &ast.Field{Names: idents, Type: typ}
   836  		params = append(params, field)
   837  		// Go spec: The scope of an identifier denoting a function
   838  		// parameter or result variable is the function body.
   839  		p.declare(field, nil, scope, ast.Var, idents...)
   840  		p.resolve(typ)
   841  		if !p.atComma("parameter list", token.RPAREN) {
   842  			return
   843  		}
   844  		p.next()
   845  		for p.tok != token.RPAREN && p.tok != token.EOF {
   846  			idents := p.parseIdentList()
   847  			typ := p.parseVarType(ellipsisOk)
   848  			field := &ast.Field{Names: idents, Type: typ}
   849  			params = append(params, field)
   850  			// Go spec: The scope of an identifier denoting a function
   851  			// parameter or result variable is the function body.
   852  			p.declare(field, nil, scope, ast.Var, idents...)
   853  			p.resolve(typ)
   854  			if !p.atComma("parameter list", token.RPAREN) {
   855  				break
   856  			}
   857  			p.next()
   858  		}
   859  		return
   860  	}
   861  
   862  	// Type { "," Type } (anonymous parameters)
   863  	params = make([]*ast.Field, len(list))
   864  	for i, typ := range list {
   865  		p.resolve(typ)
   866  		params[i] = &ast.Field{Type: typ}
   867  	}
   868  	return
   869  }
   870  
   871  func (p *parser) parseParameters(scope *ast.Scope, ellipsisOk bool) *ast.FieldList {
   872  	if p.trace {
   873  		defer un(trace(p, "Parameters"))
   874  	}
   875  
   876  	var params []*ast.Field
   877  	lparen := p.expect(token.LPAREN)
   878  	if p.tok != token.RPAREN {
   879  		params = p.parseParameterList(scope, ellipsisOk)
   880  	}
   881  	rparen := p.expect(token.RPAREN)
   882  
   883  	return &ast.FieldList{Opening: lparen, List: params, Closing: rparen}
   884  }
   885  
   886  func (p *parser) parseResult(scope *ast.Scope) *ast.FieldList {
   887  	if p.trace {
   888  		defer un(trace(p, "Result"))
   889  	}
   890  
   891  	if p.tok == token.LPAREN {
   892  		return p.parseParameters(scope, false)
   893  	}
   894  
   895  	typ := p.tryType()
   896  	if typ != nil {
   897  		list := make([]*ast.Field, 1)
   898  		list[0] = &ast.Field{Type: typ}
   899  		return &ast.FieldList{List: list}
   900  	}
   901  
   902  	return nil
   903  }
   904  
   905  func (p *parser) parseSignature(scope *ast.Scope) (params, results *ast.FieldList) {
   906  	if p.trace {
   907  		defer un(trace(p, "Signature"))
   908  	}
   909  
   910  	params = p.parseParameters(scope, true)
   911  	results = p.parseResult(scope)
   912  
   913  	return
   914  }
   915  
   916  func (p *parser) parseFuncType() (*ast.FuncType, *ast.Scope) {
   917  	if p.trace {
   918  		defer un(trace(p, "FuncType"))
   919  	}
   920  
   921  	pos := p.expect(token.FUNC)
   922  	scope := ast.NewScope(p.topScope) // function scope
   923  	params, results := p.parseSignature(scope)
   924  
   925  	return &ast.FuncType{Func: pos, Params: params, Results: results}, scope
   926  }
   927  
   928  func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
   929  	if p.trace {
   930  		defer un(trace(p, "MethodSpec"))
   931  	}
   932  
   933  	doc := p.leadComment
   934  	var idents []*ast.Ident
   935  	var typ ast.Expr
   936  	x := p.parseTypeName()
   937  	if ident, isIdent := x.(*ast.Ident); isIdent && p.tok == token.LPAREN {
   938  		// method
   939  		idents = []*ast.Ident{ident}
   940  		scope := ast.NewScope(nil) // method scope
   941  		params, results := p.parseSignature(scope)
   942  		typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
   943  	} else {
   944  		// embedded interface
   945  		typ = x
   946  		p.resolve(typ)
   947  	}
   948  	p.expectSemi() // call before accessing p.linecomment
   949  
   950  	spec := &ast.Field{Doc: doc, Names: idents, Type: typ, Comment: p.lineComment}
   951  	p.declare(spec, nil, scope, ast.Fun, idents...)
   952  
   953  	return spec
   954  }
   955  
   956  func (p *parser) parseInterfaceType() *ast.InterfaceType {
   957  	if p.trace {
   958  		defer un(trace(p, "InterfaceType"))
   959  	}
   960  
   961  	pos := p.expect(token.INTERFACE)
   962  	lbrace := p.expect(token.LBRACE)
   963  	scope := ast.NewScope(nil) // interface scope
   964  	var list []*ast.Field
   965  	for p.tok == token.IDENT {
   966  		list = append(list, p.parseMethodSpec(scope))
   967  	}
   968  	rbrace := p.expect(token.RBRACE)
   969  
   970  	return &ast.InterfaceType{
   971  		Interface: pos,
   972  		Methods: &ast.FieldList{
   973  			Opening: lbrace,
   974  			List:    list,
   975  			Closing: rbrace,
   976  		},
   977  	}
   978  }
   979  
   980  func (p *parser) parseMapType() *ast.MapType {
   981  	if p.trace {
   982  		defer un(trace(p, "MapType"))
   983  	}
   984  
   985  	pos := p.expect(token.MAP)
   986  	p.expect(token.LBRACK)
   987  	key := p.parseType()
   988  	p.expect(token.RBRACK)
   989  	value := p.parseType()
   990  
   991  	return &ast.MapType{Map: pos, Key: key, Value: value}
   992  }
   993  
   994  func (p *parser) parseChanType() *ast.ChanType {
   995  	if p.trace {
   996  		defer un(trace(p, "ChanType"))
   997  	}
   998  
   999  	pos := p.pos
  1000  	dir := ast.SEND | ast.RECV
  1001  	var arrow token.Pos
  1002  	if p.tok == token.CHAN {
  1003  		p.next()
  1004  		if p.tok == token.ARROW {
  1005  			arrow = p.pos
  1006  			p.next()
  1007  			dir = ast.SEND
  1008  		}
  1009  	} else {
  1010  		arrow = p.expect(token.ARROW)
  1011  		p.expect(token.CHAN)
  1012  		dir = ast.RECV
  1013  	}
  1014  	value := p.parseType()
  1015  
  1016  	return &ast.ChanType{Begin: pos, Arrow: arrow, Dir: dir, Value: value}
  1017  }
  1018  
  1019  // If the result is an identifier, it is not resolved.
  1020  func (p *parser) tryIdentOrType() ast.Expr {
  1021  	switch p.tok {
  1022  	case token.IDENT:
  1023  		return p.parseTypeName()
  1024  	case token.LBRACK:
  1025  		return p.parseArrayType()
  1026  	case token.STRUCT:
  1027  		return p.parseStructType()
  1028  	case token.MUL:
  1029  		return p.parsePointerType()
  1030  	case token.FUNC:
  1031  		typ, _ := p.parseFuncType()
  1032  		return typ
  1033  	case token.INTERFACE:
  1034  		return p.parseInterfaceType()
  1035  	case token.MAP:
  1036  		return p.parseMapType()
  1037  	case token.CHAN, token.ARROW:
  1038  		return p.parseChanType()
  1039  	case token.LPAREN:
  1040  		lparen := p.pos
  1041  		p.next()
  1042  		typ := p.parseType()
  1043  		rparen := p.expect(token.RPAREN)
  1044  		return &ast.ParenExpr{Lparen: lparen, X: typ, Rparen: rparen}
  1045  	}
  1046  
  1047  	// no type found
  1048  	return nil
  1049  }
  1050  
  1051  func (p *parser) tryType() ast.Expr {
  1052  	typ := p.tryIdentOrType()
  1053  	if typ != nil {
  1054  		p.resolve(typ)
  1055  	}
  1056  	return typ
  1057  }
  1058  
  1059  // ----------------------------------------------------------------------------
  1060  // Blocks
  1061  
  1062  func (p *parser) parseStmtList() (list []ast.Stmt) {
  1063  	if p.trace {
  1064  		defer un(trace(p, "StatementList"))
  1065  	}
  1066  
  1067  	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
  1068  		list = append(list, p.parseStmt())
  1069  	}
  1070  
  1071  	return
  1072  }
  1073  
  1074  func (p *parser) parseBody(scope *ast.Scope) *ast.BlockStmt {
  1075  	if p.trace {
  1076  		defer un(trace(p, "Body"))
  1077  	}
  1078  
  1079  	lbrace := p.expect(token.LBRACE)
  1080  	p.topScope = scope // open function scope
  1081  	p.openLabelScope()
  1082  	list := p.parseStmtList()
  1083  	p.closeLabelScope()
  1084  	p.closeScope()
  1085  	rbrace := p.expect(token.RBRACE)
  1086  
  1087  	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  1088  }
  1089  
  1090  func (p *parser) parseBlockStmt() *ast.BlockStmt {
  1091  	if p.trace {
  1092  		defer un(trace(p, "BlockStmt"))
  1093  	}
  1094  
  1095  	lbrace := p.expect(token.LBRACE)
  1096  	p.openScope()
  1097  	list := p.parseStmtList()
  1098  	p.closeScope()
  1099  	rbrace := p.expect(token.RBRACE)
  1100  
  1101  	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  1102  }
  1103  
  1104  // ----------------------------------------------------------------------------
  1105  // Expressions
  1106  
  1107  func (p *parser) parseFuncTypeOrLit() ast.Expr {
  1108  	if p.trace {
  1109  		defer un(trace(p, "FuncTypeOrLit"))
  1110  	}
  1111  
  1112  	typ, scope := p.parseFuncType()
  1113  	if p.tok != token.LBRACE {
  1114  		// function type only
  1115  		return typ
  1116  	}
  1117  
  1118  	p.exprLev++
  1119  	body := p.parseBody(scope)
  1120  	p.exprLev--
  1121  
  1122  	return &ast.FuncLit{Type: typ, Body: body}
  1123  }
  1124  
  1125  // parseOperand may return an expression or a raw type (incl. array
  1126  // types of the form [...]T. Callers must verify the result.
  1127  // If lhs is set and the result is an identifier, it is not resolved.
  1128  //
  1129  func (p *parser) parseOperand(lhs bool) ast.Expr {
  1130  	if p.trace {
  1131  		defer un(trace(p, "Operand"))
  1132  	}
  1133  
  1134  	switch p.tok {
  1135  	case token.IDENT:
  1136  		x := p.parseIdent()
  1137  		if !lhs {
  1138  			p.resolve(x)
  1139  		}
  1140  		return x
  1141  
  1142  	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
  1143  		x := &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
  1144  		p.next()
  1145  		return x
  1146  
  1147  	case token.LPAREN:
  1148  		lparen := p.pos
  1149  		p.next()
  1150  		p.exprLev++
  1151  		x := p.parseRhsOrType() // types may be parenthesized: (some type)
  1152  		p.exprLev--
  1153  		rparen := p.expect(token.RPAREN)
  1154  		return &ast.ParenExpr{Lparen: lparen, X: x, Rparen: rparen}
  1155  
  1156  	case token.FUNC:
  1157  		return p.parseFuncTypeOrLit()
  1158  	}
  1159  
  1160  	if typ := p.tryIdentOrType(); typ != nil {
  1161  		// could be type for composite literal or conversion
  1162  		_, isIdent := typ.(*ast.Ident)
  1163  		assert(!isIdent, "type cannot be identifier")
  1164  		return typ
  1165  	}
  1166  
  1167  	// we have an error
  1168  	pos := p.pos
  1169  	p.errorExpected(pos, "operand")
  1170  	p.advance(stmtStart)
  1171  	return &ast.BadExpr{From: pos, To: p.pos}
  1172  }
  1173  
  1174  func (p *parser) parseSelector(x ast.Expr) ast.Expr {
  1175  	if p.trace {
  1176  		defer un(trace(p, "Selector"))
  1177  	}
  1178  
  1179  	sel := p.parseIdent()
  1180  
  1181  	return &ast.SelectorExpr{X: x, Sel: sel}
  1182  }
  1183  
  1184  func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
  1185  	if p.trace {
  1186  		defer un(trace(p, "TypeAssertion"))
  1187  	}
  1188  
  1189  	lparen := p.expect(token.LPAREN)
  1190  	var typ ast.Expr
  1191  	if p.tok == token.TYPE {
  1192  		// type switch: typ == nil
  1193  		p.next()
  1194  	} else {
  1195  		typ = p.parseType()
  1196  	}
  1197  	rparen := p.expect(token.RPAREN)
  1198  
  1199  	return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
  1200  }
  1201  
  1202  func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
  1203  	if p.trace {
  1204  		defer un(trace(p, "IndexOrSlice"))
  1205  	}
  1206  
  1207  	const N = 3 // change the 3 to 2 to disable 3-index slices
  1208  	lbrack := p.expect(token.LBRACK)
  1209  	p.exprLev++
  1210  	var index [N]ast.Expr
  1211  	var colons [N - 1]token.Pos
  1212  	if p.tok != token.COLON {
  1213  		index[0] = p.parseRhs()
  1214  	}
  1215  	ncolons := 0
  1216  	for p.tok == token.COLON && ncolons < len(colons) {
  1217  		colons[ncolons] = p.pos
  1218  		ncolons++
  1219  		p.next()
  1220  		if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
  1221  			index[ncolons] = p.parseRhs()
  1222  		}
  1223  	}
  1224  	p.exprLev--
  1225  	rbrack := p.expect(token.RBRACK)
  1226  
  1227  	if ncolons > 0 {
  1228  		// slice expression
  1229  		slice3 := false
  1230  		if ncolons == 2 {
  1231  			slice3 = true
  1232  			// Check presence of 2nd and 3rd index here rather than during type-checking
  1233  			// to prevent erroneous programs from passing through gofmt (was issue 7305).
  1234  			if index[1] == nil {
  1235  				p.error(colons[0], "2nd index required in 3-index slice")
  1236  				index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
  1237  			}
  1238  			if index[2] == nil {
  1239  				p.error(colons[1], "3rd index required in 3-index slice")
  1240  				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
  1241  			}
  1242  		}
  1243  		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
  1244  	}
  1245  
  1246  	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
  1247  }
  1248  
  1249  func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
  1250  	if p.trace {
  1251  		defer un(trace(p, "CallOrConversion"))
  1252  	}
  1253  
  1254  	lparen := p.expect(token.LPAREN)
  1255  	p.exprLev++
  1256  	var list []ast.Expr
  1257  	var ellipsis token.Pos
  1258  	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
  1259  		list = append(list, p.parseRhsOrType()) // builtins may expect a type: make(some type, ...)
  1260  		if p.tok == token.ELLIPSIS {
  1261  			ellipsis = p.pos
  1262  			p.next()
  1263  		}
  1264  		if !p.atComma("argument list", token.RPAREN) {
  1265  			break
  1266  		}
  1267  		p.next()
  1268  	}
  1269  	p.exprLev--
  1270  	rparen := p.expectClosing(token.RPAREN, "argument list")
  1271  
  1272  	return &ast.CallExpr{Fun: fun, Lparen: lparen, Args: list, Ellipsis: ellipsis, Rparen: rparen}
  1273  }
  1274  
  1275  func (p *parser) parseValue(keyOk bool) ast.Expr {
  1276  	if p.trace {
  1277  		defer un(trace(p, "Element"))
  1278  	}
  1279  
  1280  	if p.tok == token.LBRACE {
  1281  		return p.parseLiteralValue(nil)
  1282  	}
  1283  
  1284  	// Because the parser doesn't know the composite literal type, it cannot
  1285  	// know if a key that's an identifier is a struct field name or a name
  1286  	// denoting a value. The former is not resolved by the parser or the
  1287  	// resolver.
  1288  	//
  1289  	// Instead, _try_ to resolve such a key if possible. If it resolves,
  1290  	// it a) has correctly resolved, or b) incorrectly resolved because
  1291  	// the key is a struct field with a name matching another identifier.
  1292  	// In the former case we are done, and in the latter case we don't
  1293  	// care because the type checker will do a separate field lookup.
  1294  	//
  1295  	// If the key does not resolve, it a) must be defined at the top
  1296  	// level in another file of the same package, the universe scope, or be
  1297  	// undeclared; or b) it is a struct field. In the former case, the type
  1298  	// checker can do a top-level lookup, and in the latter case it will do
  1299  	// a separate field lookup.
  1300  	x := p.checkExpr(p.parseExpr(keyOk))
  1301  	if keyOk {
  1302  		if p.tok == token.COLON {
  1303  			// Try to resolve the key but don't collect it
  1304  			// as unresolved identifier if it fails so that
  1305  			// we don't get (possibly false) errors about
  1306  			// undeclared names.
  1307  			p.tryResolve(x, false)
  1308  		} else {
  1309  			// not a key
  1310  			p.resolve(x)
  1311  		}
  1312  	}
  1313  
  1314  	return x
  1315  }
  1316  
  1317  func (p *parser) parseElement() ast.Expr {
  1318  	if p.trace {
  1319  		defer un(trace(p, "Element"))
  1320  	}
  1321  
  1322  	x := p.parseValue(true)
  1323  	if p.tok == token.COLON {
  1324  		colon := p.pos
  1325  		p.next()
  1326  		x = &ast.KeyValueExpr{Key: x, Colon: colon, Value: p.parseValue(false)}
  1327  	}
  1328  
  1329  	return x
  1330  }
  1331  
  1332  func (p *parser) parseElementList() (list []ast.Expr) {
  1333  	if p.trace {
  1334  		defer un(trace(p, "ElementList"))
  1335  	}
  1336  
  1337  	for p.tok != token.RBRACE && p.tok != token.EOF {
  1338  		list = append(list, p.parseElement())
  1339  		if !p.atComma("composite literal", token.RBRACE) {
  1340  			break
  1341  		}
  1342  		p.next()
  1343  	}
  1344  
  1345  	return
  1346  }
  1347  
  1348  func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
  1349  	if p.trace {
  1350  		defer un(trace(p, "LiteralValue"))
  1351  	}
  1352  
  1353  	lbrace := p.expect(token.LBRACE)
  1354  	var elts []ast.Expr
  1355  	p.exprLev++
  1356  	if p.tok != token.RBRACE {
  1357  		elts = p.parseElementList()
  1358  	}
  1359  	p.exprLev--
  1360  	rbrace := p.expectClosing(token.RBRACE, "composite literal")
  1361  	return &ast.CompositeLit{Type: typ, Lbrace: lbrace, Elts: elts, Rbrace: rbrace}
  1362  }
  1363  
  1364  // checkExpr checks that x is an expression (and not a type).
  1365  func (p *parser) checkExpr(x ast.Expr) ast.Expr {
  1366  	switch unparen(x).(type) {
  1367  	case *ast.BadExpr:
  1368  	case *ast.Ident:
  1369  	case *ast.BasicLit:
  1370  	case *ast.FuncLit:
  1371  	case *ast.CompositeLit:
  1372  	case *ast.ParenExpr:
  1373  		panic("unreachable")
  1374  	case *ast.SelectorExpr:
  1375  	case *ast.IndexExpr:
  1376  	case *ast.SliceExpr:
  1377  	case *ast.TypeAssertExpr:
  1378  		// If t.Type == nil we have a type assertion of the form
  1379  		// y.(type), which is only allowed in type switch expressions.
  1380  		// It's hard to exclude those but for the case where we are in
  1381  		// a type switch. Instead be lenient and test this in the type
  1382  		// checker.
  1383  	case *ast.CallExpr:
  1384  	case *ast.StarExpr:
  1385  	case *ast.UnaryExpr:
  1386  	case *ast.BinaryExpr:
  1387  	default:
  1388  		// all other nodes are not proper expressions
  1389  		p.errorExpected(x.Pos(), "expression")
  1390  		x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
  1391  	}
  1392  	return x
  1393  }
  1394  
  1395  // isTypeName reports whether x is a (qualified) TypeName.
  1396  func isTypeName(x ast.Expr) bool {
  1397  	switch t := x.(type) {
  1398  	case *ast.BadExpr:
  1399  	case *ast.Ident:
  1400  	case *ast.SelectorExpr:
  1401  		_, isIdent := t.X.(*ast.Ident)
  1402  		return isIdent
  1403  	default:
  1404  		return false // all other nodes are not type names
  1405  	}
  1406  	return true
  1407  }
  1408  
  1409  // isLiteralType reports whether x is a legal composite literal type.
  1410  func isLiteralType(x ast.Expr) bool {
  1411  	switch t := x.(type) {
  1412  	case *ast.BadExpr:
  1413  	case *ast.Ident:
  1414  	case *ast.SelectorExpr:
  1415  		_, isIdent := t.X.(*ast.Ident)
  1416  		return isIdent
  1417  	case *ast.ArrayType:
  1418  	case *ast.StructType:
  1419  	case *ast.MapType:
  1420  	default:
  1421  		return false // all other nodes are not legal composite literal types
  1422  	}
  1423  	return true
  1424  }
  1425  
  1426  // If x is of the form *T, deref returns T, otherwise it returns x.
  1427  func deref(x ast.Expr) ast.Expr {
  1428  	if p, isPtr := x.(*ast.StarExpr); isPtr {
  1429  		x = p.X
  1430  	}
  1431  	return x
  1432  }
  1433  
  1434  // If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
  1435  func unparen(x ast.Expr) ast.Expr {
  1436  	if p, isParen := x.(*ast.ParenExpr); isParen {
  1437  		x = unparen(p.X)
  1438  	}
  1439  	return x
  1440  }
  1441  
  1442  // checkExprOrType checks that x is an expression or a type
  1443  // (and not a raw type such as [...]T).
  1444  //
  1445  func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
  1446  	switch t := unparen(x).(type) {
  1447  	case *ast.ParenExpr:
  1448  		panic("unreachable")
  1449  	case *ast.UnaryExpr:
  1450  	case *ast.ArrayType:
  1451  		if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
  1452  			p.error(len.Pos(), "expected array length, found '...'")
  1453  			x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
  1454  		}
  1455  	}
  1456  
  1457  	// all other nodes are expressions or types
  1458  	return x
  1459  }
  1460  
  1461  // If lhs is set and the result is an identifier, it is not resolved.
  1462  func (p *parser) parsePrimaryExpr(lhs bool) ast.Expr {
  1463  	if p.trace {
  1464  		defer un(trace(p, "PrimaryExpr"))
  1465  	}
  1466  
  1467  	x := p.parseOperand(lhs)
  1468  L:
  1469  	for {
  1470  		switch p.tok {
  1471  		case token.PERIOD:
  1472  			p.next()
  1473  			if lhs {
  1474  				p.resolve(x)
  1475  			}
  1476  			switch p.tok {
  1477  			case token.IDENT:
  1478  				x = p.parseSelector(p.checkExprOrType(x))
  1479  			case token.LPAREN:
  1480  				x = p.parseTypeAssertion(p.checkExpr(x))
  1481  			default:
  1482  				pos := p.pos
  1483  				p.errorExpected(pos, "selector or type assertion")
  1484  				p.next() // make progress
  1485  				sel := &ast.Ident{NamePos: pos, Name: "_"}
  1486  				x = &ast.SelectorExpr{X: x, Sel: sel}
  1487  			}
  1488  		case token.LBRACK:
  1489  			if lhs {
  1490  				p.resolve(x)
  1491  			}
  1492  			x = p.parseIndexOrSlice(p.checkExpr(x))
  1493  		case token.LPAREN:
  1494  			if lhs {
  1495  				p.resolve(x)
  1496  			}
  1497  			x = p.parseCallOrConversion(p.checkExprOrType(x))
  1498  		case token.LBRACE:
  1499  			if isLiteralType(x) && (p.exprLev >= 0 || !isTypeName(x)) {
  1500  				if lhs {
  1501  					p.resolve(x)
  1502  				}
  1503  				x = p.parseLiteralValue(x)
  1504  			} else {
  1505  				break L
  1506  			}
  1507  		default:
  1508  			break L
  1509  		}
  1510  		lhs = false // no need to try to resolve again
  1511  	}
  1512  
  1513  	return x
  1514  }
  1515  
  1516  // If lhs is set and the result is an identifier, it is not resolved.
  1517  func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
  1518  	if p.trace {
  1519  		defer un(trace(p, "UnaryExpr"))
  1520  	}
  1521  
  1522  	switch p.tok {
  1523  	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND:
  1524  		pos, op := p.pos, p.tok
  1525  		p.next()
  1526  		x := p.parseUnaryExpr(false)
  1527  		return &ast.UnaryExpr{OpPos: pos, Op: op, X: p.checkExpr(x)}
  1528  
  1529  	case token.ARROW:
  1530  		// channel type or receive expression
  1531  		arrow := p.pos
  1532  		p.next()
  1533  
  1534  		// If the next token is token.CHAN we still don't know if it
  1535  		// is a channel type or a receive operation - we only know
  1536  		// once we have found the end of the unary expression. There
  1537  		// are two cases:
  1538  		//
  1539  		//   <- type  => (<-type) must be channel type
  1540  		//   <- expr  => <-(expr) is a receive from an expression
  1541  		//
  1542  		// In the first case, the arrow must be re-associated with
  1543  		// the channel type parsed already:
  1544  		//
  1545  		//   <- (chan type)    =>  (<-chan type)
  1546  		//   <- (chan<- type)  =>  (<-chan (<-type))
  1547  
  1548  		x := p.parseUnaryExpr(false)
  1549  
  1550  		// determine which case we have
  1551  		if typ, ok := x.(*ast.ChanType); ok {
  1552  			// (<-type)
  1553  
  1554  			// re-associate position info and <-
  1555  			dir := ast.SEND
  1556  			for ok && dir == ast.SEND {
  1557  				if typ.Dir == ast.RECV {
  1558  					// error: (<-type) is (<-(<-chan T))
  1559  					p.errorExpected(typ.Arrow, "'chan'")
  1560  				}
  1561  				arrow, typ.Begin, typ.Arrow = typ.Arrow, arrow, arrow
  1562  				dir, typ.Dir = typ.Dir, ast.RECV
  1563  				typ, ok = typ.Value.(*ast.ChanType)
  1564  			}
  1565  			if dir == ast.SEND {
  1566  				p.errorExpected(arrow, "channel type")
  1567  			}
  1568  
  1569  			return x
  1570  		}
  1571  
  1572  		// <-(expr)
  1573  		return &ast.UnaryExpr{OpPos: arrow, Op: token.ARROW, X: p.checkExpr(x)}
  1574  
  1575  	case token.MUL:
  1576  		// pointer type or unary "*" expression
  1577  		pos := p.pos
  1578  		p.next()
  1579  		x := p.parseUnaryExpr(false)
  1580  		return &ast.StarExpr{Star: pos, X: p.checkExprOrType(x)}
  1581  	}
  1582  
  1583  	return p.parsePrimaryExpr(lhs)
  1584  }
  1585  
  1586  func (p *parser) tokPrec() (token.Token, int) {
  1587  	tok := p.tok
  1588  	if p.inRhs && tok == token.ASSIGN {
  1589  		tok = token.EQL
  1590  	}
  1591  	return tok, tok.Precedence()
  1592  }
  1593  
  1594  // If lhs is set and the result is an identifier, it is not resolved.
  1595  func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
  1596  	if p.trace {
  1597  		defer un(trace(p, "BinaryExpr"))
  1598  	}
  1599  
  1600  	x := p.parseUnaryExpr(lhs)
  1601  	for {
  1602  		op, oprec := p.tokPrec()
  1603  		if oprec < prec1 {
  1604  			return x
  1605  		}
  1606  		pos := p.expect(op)
  1607  		if lhs {
  1608  			p.resolve(x)
  1609  			lhs = false
  1610  		}
  1611  		y := p.parseBinaryExpr(false, oprec+1)
  1612  		x = &ast.BinaryExpr{X: p.checkExpr(x), OpPos: pos, Op: op, Y: p.checkExpr(y)}
  1613  	}
  1614  }
  1615  
  1616  // If lhs is set and the result is an identifier, it is not resolved.
  1617  // The result may be a type or even a raw type ([...]int). Callers must
  1618  // check the result (using checkExpr or checkExprOrType), depending on
  1619  // context.
  1620  func (p *parser) parseExpr(lhs bool) ast.Expr {
  1621  	if p.trace {
  1622  		defer un(trace(p, "Expression"))
  1623  	}
  1624  
  1625  	return p.parseBinaryExpr(lhs, token.LowestPrec+1)
  1626  }
  1627  
  1628  func (p *parser) parseRhs() ast.Expr {
  1629  	old := p.inRhs
  1630  	p.inRhs = true
  1631  	x := p.checkExpr(p.parseExpr(false))
  1632  	p.inRhs = old
  1633  	return x
  1634  }
  1635  
  1636  func (p *parser) parseRhsOrType() ast.Expr {
  1637  	old := p.inRhs
  1638  	p.inRhs = true
  1639  	x := p.checkExprOrType(p.parseExpr(false))
  1640  	p.inRhs = old
  1641  	return x
  1642  }
  1643  
  1644  // ----------------------------------------------------------------------------
  1645  // Statements
  1646  
  1647  // Parsing modes for parseSimpleStmt.
  1648  const (
  1649  	basic = iota
  1650  	labelOk
  1651  	rangeOk
  1652  )
  1653  
  1654  // parseSimpleStmt returns true as 2nd result if it parsed the assignment
  1655  // of a range clause (with mode == rangeOk). The returned statement is an
  1656  // assignment with a right-hand side that is a single unary expression of
  1657  // the form "range x". No guarantees are given for the left-hand side.
  1658  func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
  1659  	if p.trace {
  1660  		defer un(trace(p, "SimpleStmt"))
  1661  	}
  1662  
  1663  	x := p.parseLhsList()
  1664  
  1665  	switch p.tok {
  1666  	case
  1667  		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
  1668  		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
  1669  		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
  1670  		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
  1671  		// assignment statement, possibly part of a range clause
  1672  		pos, tok := p.pos, p.tok
  1673  		p.next()
  1674  		var y []ast.Expr
  1675  		isRange := false
  1676  		if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
  1677  			pos := p.pos
  1678  			p.next()
  1679  			y = []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
  1680  			isRange = true
  1681  		} else {
  1682  			y = p.parseRhsList()
  1683  		}
  1684  		as := &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}
  1685  		if tok == token.DEFINE {
  1686  			p.shortVarDecl(as, x)
  1687  		}
  1688  		return as, isRange
  1689  	}
  1690  
  1691  	if len(x) > 1 {
  1692  		p.errorExpected(x[0].Pos(), "1 expression")
  1693  		// continue with first expression
  1694  	}
  1695  
  1696  	switch p.tok {
  1697  	case token.COLON:
  1698  		// labeled statement
  1699  		colon := p.pos
  1700  		p.next()
  1701  		if label, isIdent := x[0].(*ast.Ident); mode == labelOk && isIdent {
  1702  			// Go spec: The scope of a label is the body of the function
  1703  			// in which it is declared and excludes the body of any nested
  1704  			// function.
  1705  			stmt := &ast.LabeledStmt{Label: label, Colon: colon, Stmt: p.parseStmt()}
  1706  			p.declare(stmt, nil, p.labelScope, ast.Lbl, label)
  1707  			return stmt, false
  1708  		}
  1709  		// The label declaration typically starts at x[0].Pos(), but the label
  1710  		// declaration may be erroneous due to a token after that position (and
  1711  		// before the ':'). If SpuriousErrors is not set, the (only) error
  1712  		// reported for the line is the illegal label error instead of the token
  1713  		// before the ':' that caused the problem. Thus, use the (latest) colon
  1714  		// position for error reporting.
  1715  		p.error(colon, "illegal label declaration")
  1716  		return &ast.BadStmt{From: x[0].Pos(), To: colon + 1}, false
  1717  
  1718  	case token.ARROW:
  1719  		// send statement
  1720  		arrow := p.pos
  1721  		p.next()
  1722  		y := p.parseRhs()
  1723  		return &ast.SendStmt{Chan: x[0], Arrow: arrow, Value: y}, false
  1724  
  1725  	case token.INC, token.DEC:
  1726  		// increment or decrement
  1727  		s := &ast.IncDecStmt{X: x[0], TokPos: p.pos, Tok: p.tok}
  1728  		p.next()
  1729  		return s, false
  1730  	}
  1731  
  1732  	// expression
  1733  	return &ast.ExprStmt{X: x[0]}, false
  1734  }
  1735  
  1736  func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
  1737  	x := p.parseRhsOrType() // could be a conversion: (some type)(x)
  1738  	if call, isCall := x.(*ast.CallExpr); isCall {
  1739  		return call
  1740  	}
  1741  	if _, isBad := x.(*ast.BadExpr); !isBad {
  1742  		// only report error if it's a new one
  1743  		p.error(p.safePos(x.End()), fmt.Sprintf("function must be invoked in %s statement", callType))
  1744  	}
  1745  	return nil
  1746  }
  1747  
  1748  func (p *parser) parseGoStmt() ast.Stmt {
  1749  	if p.trace {
  1750  		defer un(trace(p, "GoStmt"))
  1751  	}
  1752  
  1753  	pos := p.expect(token.GO)
  1754  	call := p.parseCallExpr("go")
  1755  	p.expectSemi()
  1756  	if call == nil {
  1757  		return &ast.BadStmt{From: pos, To: pos + 2} // len("go")
  1758  	}
  1759  
  1760  	return &ast.GoStmt{Go: pos, Call: call}
  1761  }
  1762  
  1763  func (p *parser) parseDeferStmt() ast.Stmt {
  1764  	if p.trace {
  1765  		defer un(trace(p, "DeferStmt"))
  1766  	}
  1767  
  1768  	pos := p.expect(token.DEFER)
  1769  	call := p.parseCallExpr("defer")
  1770  	p.expectSemi()
  1771  	if call == nil {
  1772  		return &ast.BadStmt{From: pos, To: pos + 5} // len("defer")
  1773  	}
  1774  
  1775  	return &ast.DeferStmt{Defer: pos, Call: call}
  1776  }
  1777  
  1778  func (p *parser) parseReturnStmt() *ast.ReturnStmt {
  1779  	if p.trace {
  1780  		defer un(trace(p, "ReturnStmt"))
  1781  	}
  1782  
  1783  	pos := p.pos
  1784  	p.expect(token.RETURN)
  1785  	var x []ast.Expr
  1786  	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
  1787  		x = p.parseRhsList()
  1788  	}
  1789  	p.expectSemi()
  1790  
  1791  	return &ast.ReturnStmt{Return: pos, Results: x}
  1792  }
  1793  
  1794  func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
  1795  	if p.trace {
  1796  		defer un(trace(p, "BranchStmt"))
  1797  	}
  1798  
  1799  	pos := p.expect(tok)
  1800  	var label *ast.Ident
  1801  	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
  1802  		label = p.parseIdent()
  1803  		// add to list of unresolved targets
  1804  		n := len(p.targetStack) - 1
  1805  		p.targetStack[n] = append(p.targetStack[n], label)
  1806  	}
  1807  	p.expectSemi()
  1808  
  1809  	return &ast.BranchStmt{TokPos: pos, Tok: tok, Label: label}
  1810  }
  1811  
  1812  func (p *parser) makeExpr(s ast.Stmt, want string) ast.Expr {
  1813  	if s == nil {
  1814  		return nil
  1815  	}
  1816  	if es, isExpr := s.(*ast.ExprStmt); isExpr {
  1817  		return p.checkExpr(es.X)
  1818  	}
  1819  	found := "simple statement"
  1820  	if _, isAss := s.(*ast.AssignStmt); isAss {
  1821  		found = "assignment"
  1822  	}
  1823  	p.error(s.Pos(), fmt.Sprintf("expected %s, found %s (missing parentheses around composite literal?)", want, found))
  1824  	return &ast.BadExpr{From: s.Pos(), To: p.safePos(s.End())}
  1825  }
  1826  
  1827  // parseIfHeader is an adjusted version of parser.header
  1828  // in cmd/compile/internal/syntax/parser.go, which has
  1829  // been tuned for better error handling.
  1830  func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
  1831  	if p.tok == token.LBRACE {
  1832  		p.error(p.pos, "missing condition in if statement")
  1833  		cond = &ast.BadExpr{From: p.pos, To: p.pos}
  1834  		return
  1835  	}
  1836  	// p.tok != token.LBRACE
  1837  
  1838  	outer := p.exprLev
  1839  	p.exprLev = -1
  1840  
  1841  	if p.tok != token.SEMICOLON {
  1842  		// accept potential variable declaration but complain
  1843  		if p.tok == token.VAR {
  1844  			p.next()
  1845  			p.error(p.pos, fmt.Sprintf("var declaration not allowed in 'IF' initializer"))
  1846  		}
  1847  		init, _ = p.parseSimpleStmt(basic)
  1848  	}
  1849  
  1850  	var condStmt ast.Stmt
  1851  	var semi struct {
  1852  		pos token.Pos
  1853  		lit string // ";" or "\n"; valid if pos.IsValid()
  1854  	}
  1855  	if p.tok != token.LBRACE {
  1856  		if p.tok == token.SEMICOLON {
  1857  			semi.pos = p.pos
  1858  			semi.lit = p.lit
  1859  			p.next()
  1860  		} else {
  1861  			p.expect(token.SEMICOLON)
  1862  		}
  1863  		if p.tok != token.LBRACE {
  1864  			condStmt, _ = p.parseSimpleStmt(basic)
  1865  		}
  1866  	} else {
  1867  		condStmt = init
  1868  		init = nil
  1869  	}
  1870  
  1871  	if condStmt != nil {
  1872  		cond = p.makeExpr(condStmt, "boolean expression")
  1873  	} else if semi.pos.IsValid() {
  1874  		if semi.lit == "\n" {
  1875  			p.error(semi.pos, "unexpected newline, expecting { after if clause")
  1876  		} else {
  1877  			p.error(semi.pos, "missing condition in if statement")
  1878  		}
  1879  	}
  1880  
  1881  	// make sure we have a valid AST
  1882  	if cond == nil {
  1883  		cond = &ast.BadExpr{From: p.pos, To: p.pos}
  1884  	}
  1885  
  1886  	p.exprLev = outer
  1887  	return
  1888  }
  1889  
  1890  func (p *parser) parseIfStmt() *ast.IfStmt {
  1891  	if p.trace {
  1892  		defer un(trace(p, "IfStmt"))
  1893  	}
  1894  
  1895  	pos := p.expect(token.IF)
  1896  	p.openScope()
  1897  	defer p.closeScope()
  1898  
  1899  	init, cond := p.parseIfHeader()
  1900  	body := p.parseBlockStmt()
  1901  
  1902  	var else_ ast.Stmt
  1903  	if p.tok == token.ELSE {
  1904  		p.next()
  1905  		switch p.tok {
  1906  		case token.IF:
  1907  			else_ = p.parseIfStmt()
  1908  		case token.LBRACE:
  1909  			else_ = p.parseBlockStmt()
  1910  			p.expectSemi()
  1911  		default:
  1912  			p.errorExpected(p.pos, "if statement or block")
  1913  			else_ = &ast.BadStmt{From: p.pos, To: p.pos}
  1914  		}
  1915  	} else {
  1916  		p.expectSemi()
  1917  	}
  1918  
  1919  	return &ast.IfStmt{If: pos, Init: init, Cond: cond, Body: body, Else: else_}
  1920  }
  1921  
  1922  func (p *parser) parseTypeList() (list []ast.Expr) {
  1923  	if p.trace {
  1924  		defer un(trace(p, "TypeList"))
  1925  	}
  1926  
  1927  	list = append(list, p.parseType())
  1928  	for p.tok == token.COMMA {
  1929  		p.next()
  1930  		list = append(list, p.parseType())
  1931  	}
  1932  
  1933  	return
  1934  }
  1935  
  1936  func (p *parser) parseCaseClause(typeSwitch bool) *ast.CaseClause {
  1937  	if p.trace {
  1938  		defer un(trace(p, "CaseClause"))
  1939  	}
  1940  
  1941  	pos := p.pos
  1942  	var list []ast.Expr
  1943  	if p.tok == token.CASE {
  1944  		p.next()
  1945  		if typeSwitch {
  1946  			list = p.parseTypeList()
  1947  		} else {
  1948  			list = p.parseRhsList()
  1949  		}
  1950  	} else {
  1951  		p.expect(token.DEFAULT)
  1952  	}
  1953  
  1954  	colon := p.expect(token.COLON)
  1955  	p.openScope()
  1956  	body := p.parseStmtList()
  1957  	p.closeScope()
  1958  
  1959  	return &ast.CaseClause{Case: pos, List: list, Colon: colon, Body: body}
  1960  }
  1961  
  1962  func isTypeSwitchAssert(x ast.Expr) bool {
  1963  	a, ok := x.(*ast.TypeAssertExpr)
  1964  	return ok && a.Type == nil
  1965  }
  1966  
  1967  func (p *parser) isTypeSwitchGuard(s ast.Stmt) bool {
  1968  	switch t := s.(type) {
  1969  	case *ast.ExprStmt:
  1970  		// x.(type)
  1971  		return isTypeSwitchAssert(t.X)
  1972  	case *ast.AssignStmt:
  1973  		// v := x.(type)
  1974  		if len(t.Lhs) == 1 && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0]) {
  1975  			switch t.Tok {
  1976  			case token.ASSIGN:
  1977  				// permit v = x.(type) but complain
  1978  				p.error(t.TokPos, "expected ':=', found '='")
  1979  				fallthrough
  1980  			case token.DEFINE:
  1981  				return true
  1982  			}
  1983  		}
  1984  	}
  1985  	return false
  1986  }
  1987  
  1988  func (p *parser) parseSwitchStmt() ast.Stmt {
  1989  	if p.trace {
  1990  		defer un(trace(p, "SwitchStmt"))
  1991  	}
  1992  
  1993  	pos := p.expect(token.SWITCH)
  1994  	p.openScope()
  1995  	defer p.closeScope()
  1996  
  1997  	var s1, s2 ast.Stmt
  1998  	if p.tok != token.LBRACE {
  1999  		prevLev := p.exprLev
  2000  		p.exprLev = -1
  2001  		if p.tok != token.SEMICOLON {
  2002  			s2, _ = p.parseSimpleStmt(basic)
  2003  		}
  2004  		if p.tok == token.SEMICOLON {
  2005  			p.next()
  2006  			s1 = s2
  2007  			s2 = nil
  2008  			if p.tok != token.LBRACE {
  2009  				// A TypeSwitchGuard may declare a variable in addition
  2010  				// to the variable declared in the initial SimpleStmt.
  2011  				// Introduce extra scope to avoid redeclaration errors:
  2012  				//
  2013  				//	switch t := 0; t := x.(T) { ... }
  2014  				//
  2015  				// (this code is not valid Go because the first t
  2016  				// cannot be accessed and thus is never used, the extra
  2017  				// scope is needed for the correct error message).
  2018  				//
  2019  				// If we don't have a type switch, s2 must be an expression.
  2020  				// Having the extra nested but empty scope won't affect it.
  2021  				p.openScope()
  2022  				defer p.closeScope()
  2023  				s2, _ = p.parseSimpleStmt(basic)
  2024  			}
  2025  		}
  2026  		p.exprLev = prevLev
  2027  	}
  2028  
  2029  	typeSwitch := p.isTypeSwitchGuard(s2)
  2030  	lbrace := p.expect(token.LBRACE)
  2031  	var list []ast.Stmt
  2032  	for p.tok == token.CASE || p.tok == token.DEFAULT {
  2033  		list = append(list, p.parseCaseClause(typeSwitch))
  2034  	}
  2035  	rbrace := p.expect(token.RBRACE)
  2036  	p.expectSemi()
  2037  	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  2038  
  2039  	if typeSwitch {
  2040  		return &ast.TypeSwitchStmt{Switch: pos, Init: s1, Assign: s2, Body: body}
  2041  	}
  2042  
  2043  	return &ast.SwitchStmt{Switch: pos, Init: s1, Tag: p.makeExpr(s2, "switch expression"), Body: body}
  2044  }
  2045  
  2046  func (p *parser) parseCommClause() *ast.CommClause {
  2047  	if p.trace {
  2048  		defer un(trace(p, "CommClause"))
  2049  	}
  2050  
  2051  	p.openScope()
  2052  	pos := p.pos
  2053  	var comm ast.Stmt
  2054  	if p.tok == token.CASE {
  2055  		p.next()
  2056  		lhs := p.parseLhsList()
  2057  		if p.tok == token.ARROW {
  2058  			// SendStmt
  2059  			if len(lhs) > 1 {
  2060  				p.errorExpected(lhs[0].Pos(), "1 expression")
  2061  				// continue with first expression
  2062  			}
  2063  			arrow := p.pos
  2064  			p.next()
  2065  			rhs := p.parseRhs()
  2066  			comm = &ast.SendStmt{Chan: lhs[0], Arrow: arrow, Value: rhs}
  2067  		} else {
  2068  			// RecvStmt
  2069  			if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
  2070  				// RecvStmt with assignment
  2071  				if len(lhs) > 2 {
  2072  					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
  2073  					// continue with first two expressions
  2074  					lhs = lhs[0:2]
  2075  				}
  2076  				pos := p.pos
  2077  				p.next()
  2078  				rhs := p.parseRhs()
  2079  				as := &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
  2080  				if tok == token.DEFINE {
  2081  					p.shortVarDecl(as, lhs)
  2082  				}
  2083  				comm = as
  2084  			} else {
  2085  				// lhs must be single receive operation
  2086  				if len(lhs) > 1 {
  2087  					p.errorExpected(lhs[0].Pos(), "1 expression")
  2088  					// continue with first expression
  2089  				}
  2090  				comm = &ast.ExprStmt{X: lhs[0]}
  2091  			}
  2092  		}
  2093  	} else {
  2094  		p.expect(token.DEFAULT)
  2095  	}
  2096  
  2097  	colon := p.expect(token.COLON)
  2098  	body := p.parseStmtList()
  2099  	p.closeScope()
  2100  
  2101  	return &ast.CommClause{Case: pos, Comm: comm, Colon: colon, Body: body}
  2102  }
  2103  
  2104  func (p *parser) parseSelectStmt() *ast.SelectStmt {
  2105  	if p.trace {
  2106  		defer un(trace(p, "SelectStmt"))
  2107  	}
  2108  
  2109  	pos := p.expect(token.SELECT)
  2110  	lbrace := p.expect(token.LBRACE)
  2111  	var list []ast.Stmt
  2112  	for p.tok == token.CASE || p.tok == token.DEFAULT {
  2113  		list = append(list, p.parseCommClause())
  2114  	}
  2115  	rbrace := p.expect(token.RBRACE)
  2116  	p.expectSemi()
  2117  	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  2118  
  2119  	return &ast.SelectStmt{Select: pos, Body: body}
  2120  }
  2121  
  2122  func (p *parser) parseForStmt() ast.Stmt {
  2123  	if p.trace {
  2124  		defer un(trace(p, "ForStmt"))
  2125  	}
  2126  
  2127  	pos := p.expect(token.FOR)
  2128  	p.openScope()
  2129  	defer p.closeScope()
  2130  
  2131  	var s1, s2, s3 ast.Stmt
  2132  	var isRange bool
  2133  	if p.tok != token.LBRACE {
  2134  		prevLev := p.exprLev
  2135  		p.exprLev = -1
  2136  		if p.tok != token.SEMICOLON {
  2137  			if p.tok == token.RANGE {
  2138  				// "for range x" (nil lhs in assignment)
  2139  				pos := p.pos
  2140  				p.next()
  2141  				y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
  2142  				s2 = &ast.AssignStmt{Rhs: y}
  2143  				isRange = true
  2144  			} else {
  2145  				s2, isRange = p.parseSimpleStmt(rangeOk)
  2146  			}
  2147  		}
  2148  		if !isRange && p.tok == token.SEMICOLON {
  2149  			p.next()
  2150  			s1 = s2
  2151  			s2 = nil
  2152  			if p.tok != token.SEMICOLON {
  2153  				s2, _ = p.parseSimpleStmt(basic)
  2154  			}
  2155  			p.expectSemi()
  2156  			if p.tok != token.LBRACE {
  2157  				s3, _ = p.parseSimpleStmt(basic)
  2158  			}
  2159  		}
  2160  		p.exprLev = prevLev
  2161  	}
  2162  
  2163  	body := p.parseBlockStmt()
  2164  	p.expectSemi()
  2165  
  2166  	if isRange {
  2167  		as := s2.(*ast.AssignStmt)
  2168  		// check lhs
  2169  		var key, value ast.Expr
  2170  		switch len(as.Lhs) {
  2171  		case 0:
  2172  			// nothing to do
  2173  		case 1:
  2174  			key = as.Lhs[0]
  2175  		case 2:
  2176  			key, value = as.Lhs[0], as.Lhs[1]
  2177  		default:
  2178  			p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
  2179  			return &ast.BadStmt{From: pos, To: p.safePos(body.End())}
  2180  		}
  2181  		// parseSimpleStmt returned a right-hand side that
  2182  		// is a single unary expression of the form "range x"
  2183  		x := as.Rhs[0].(*ast.UnaryExpr).X
  2184  		return &ast.RangeStmt{
  2185  			For:    pos,
  2186  			Key:    key,
  2187  			Value:  value,
  2188  			TokPos: as.TokPos,
  2189  			Tok:    as.Tok,
  2190  			X:      x,
  2191  			Body:   body,
  2192  		}
  2193  	}
  2194  
  2195  	// regular for statement
  2196  	return &ast.ForStmt{
  2197  		For:  pos,
  2198  		Init: s1,
  2199  		Cond: p.makeExpr(s2, "boolean or range expression"),
  2200  		Post: s3,
  2201  		Body: body,
  2202  	}
  2203  }
  2204  
  2205  func (p *parser) parseStmt() (s ast.Stmt) {
  2206  	if p.trace {
  2207  		defer un(trace(p, "Statement"))
  2208  	}
  2209  
  2210  	switch p.tok {
  2211  	case token.CONST, token.TYPE, token.VAR:
  2212  		s = &ast.DeclStmt{Decl: p.parseDecl(stmtStart)}
  2213  	case
  2214  		// tokens that may start an expression
  2215  		token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
  2216  		token.LBRACK, token.STRUCT, token.MAP, token.CHAN, token.INTERFACE, // composite types
  2217  		token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators
  2218  		s, _ = p.parseSimpleStmt(labelOk)
  2219  		// because of the required look-ahead, labeled statements are
  2220  		// parsed by parseSimpleStmt - don't expect a semicolon after
  2221  		// them
  2222  		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
  2223  			p.expectSemi()
  2224  		}
  2225  	case token.GO:
  2226  		s = p.parseGoStmt()
  2227  	case token.DEFER:
  2228  		s = p.parseDeferStmt()
  2229  	case token.RETURN:
  2230  		s = p.parseReturnStmt()
  2231  	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
  2232  		s = p.parseBranchStmt(p.tok)
  2233  	case token.LBRACE:
  2234  		s = p.parseBlockStmt()
  2235  		p.expectSemi()
  2236  	case token.IF:
  2237  		s = p.parseIfStmt()
  2238  	case token.SWITCH:
  2239  		s = p.parseSwitchStmt()
  2240  	case token.SELECT:
  2241  		s = p.parseSelectStmt()
  2242  	case token.FOR:
  2243  		s = p.parseForStmt()
  2244  	case token.SEMICOLON:
  2245  		// Is it ever possible to have an implicit semicolon
  2246  		// producing an empty statement in a valid program?
  2247  		// (handle correctly anyway)
  2248  		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: p.lit == "\n"}
  2249  		p.next()
  2250  	case token.RBRACE:
  2251  		// a semicolon may be omitted before a closing "}"
  2252  		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: true}
  2253  	default:
  2254  		// no statement found
  2255  		pos := p.pos
  2256  		p.errorExpected(pos, "statement")
  2257  		p.advance(stmtStart)
  2258  		s = &ast.BadStmt{From: pos, To: p.pos}
  2259  	}
  2260  
  2261  	return
  2262  }
  2263  
  2264  // ----------------------------------------------------------------------------
  2265  // Declarations
  2266  
  2267  type parseSpecFunction func(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec
  2268  
  2269  func isValidImport(lit string) bool {
  2270  	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
  2271  	s, _ := strconv.Unquote(lit) // go/scanner returns a legal string literal
  2272  	for _, r := range s {
  2273  		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
  2274  			return false
  2275  		}
  2276  	}
  2277  	return s != ""
  2278  }
  2279  
  2280  func (p *parser) parseImportSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
  2281  	if p.trace {
  2282  		defer un(trace(p, "ImportSpec"))
  2283  	}
  2284  
  2285  	var ident *ast.Ident
  2286  	switch p.tok {
  2287  	case token.PERIOD:
  2288  		ident = &ast.Ident{NamePos: p.pos, Name: "."}
  2289  		p.next()
  2290  	case token.IDENT:
  2291  		ident = p.parseIdent()
  2292  	}
  2293  
  2294  	pos := p.pos
  2295  	var path string
  2296  	if p.tok == token.STRING {
  2297  		path = p.lit
  2298  		if !isValidImport(path) {
  2299  			p.error(pos, "invalid import path: "+path)
  2300  		}
  2301  		p.next()
  2302  	} else {
  2303  		p.expect(token.STRING) // use expect() error handling
  2304  	}
  2305  	p.expectSemi() // call before accessing p.linecomment
  2306  
  2307  	// collect imports
  2308  	spec := &ast.ImportSpec{
  2309  		Doc:     doc,
  2310  		Name:    ident,
  2311  		Path:    &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: path},
  2312  		Comment: p.lineComment,
  2313  	}
  2314  	p.imports = append(p.imports, spec)
  2315  
  2316  	return spec
  2317  }
  2318  
  2319  func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec {
  2320  	if p.trace {
  2321  		defer un(trace(p, keyword.String()+"Spec"))
  2322  	}
  2323  
  2324  	pos := p.pos
  2325  	idents := p.parseIdentList()
  2326  	typ := p.tryType()
  2327  	var values []ast.Expr
  2328  	// always permit optional initialization for more tolerant parsing
  2329  	if p.tok == token.ASSIGN {
  2330  		p.next()
  2331  		values = p.parseRhsList()
  2332  	}
  2333  	p.expectSemi() // call before accessing p.linecomment
  2334  
  2335  	switch keyword {
  2336  	case token.VAR:
  2337  		if typ == nil && values == nil {
  2338  			p.error(pos, "missing variable type or initialization")
  2339  		}
  2340  	case token.CONST:
  2341  		if values == nil && (iota == 0 || typ != nil) {
  2342  			p.error(pos, "missing constant value")
  2343  		}
  2344  	}
  2345  
  2346  	// Go spec: The scope of a constant or variable identifier declared inside
  2347  	// a function begins at the end of the ConstSpec or VarSpec and ends at
  2348  	// the end of the innermost containing block.
  2349  	// (Global identifiers are resolved in a separate phase after parsing.)
  2350  	spec := &ast.ValueSpec{
  2351  		Doc:     doc,
  2352  		Names:   idents,
  2353  		Type:    typ,
  2354  		Values:  values,
  2355  		Comment: p.lineComment,
  2356  	}
  2357  	kind := ast.Con
  2358  	if keyword == token.VAR {
  2359  		kind = ast.Var
  2360  	}
  2361  	p.declare(spec, iota, p.topScope, kind, idents...)
  2362  
  2363  	return spec
  2364  }
  2365  
  2366  func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
  2367  	if p.trace {
  2368  		defer un(trace(p, "TypeSpec"))
  2369  	}
  2370  
  2371  	ident := p.parseIdent()
  2372  
  2373  	// Go spec: The scope of a type identifier declared inside a function begins
  2374  	// at the identifier in the TypeSpec and ends at the end of the innermost
  2375  	// containing block.
  2376  	// (Global identifiers are resolved in a separate phase after parsing.)
  2377  	spec := &ast.TypeSpec{Doc: doc, Name: ident}
  2378  	p.declare(spec, nil, p.topScope, ast.Typ, ident)
  2379  	if p.tok == token.ASSIGN {
  2380  		spec.Assign = p.pos
  2381  		p.next()
  2382  	}
  2383  	spec.Type = p.parseType()
  2384  	p.expectSemi() // call before accessing p.linecomment
  2385  	spec.Comment = p.lineComment
  2386  
  2387  	return spec
  2388  }
  2389  
  2390  func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
  2391  	if p.trace {
  2392  		defer un(trace(p, "GenDecl("+keyword.String()+")"))
  2393  	}
  2394  
  2395  	doc := p.leadComment
  2396  	pos := p.expect(keyword)
  2397  	var lparen, rparen token.Pos
  2398  	var list []ast.Spec
  2399  	if p.tok == token.LPAREN {
  2400  		lparen = p.pos
  2401  		p.next()
  2402  		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
  2403  			list = append(list, f(p.leadComment, keyword, iota))
  2404  		}
  2405  		rparen = p.expect(token.RPAREN)
  2406  		p.expectSemi()
  2407  	} else {
  2408  		list = append(list, f(nil, keyword, 0))
  2409  	}
  2410  
  2411  	return &ast.GenDecl{
  2412  		Doc:    doc,
  2413  		TokPos: pos,
  2414  		Tok:    keyword,
  2415  		Lparen: lparen,
  2416  		Specs:  list,
  2417  		Rparen: rparen,
  2418  	}
  2419  }
  2420  
  2421  func (p *parser) parseFuncDecl() *ast.FuncDecl {
  2422  	if p.trace {
  2423  		defer un(trace(p, "FunctionDecl"))
  2424  	}
  2425  
  2426  	doc := p.leadComment
  2427  	pos := p.expect(token.FUNC)
  2428  	scope := ast.NewScope(p.topScope) // function scope
  2429  
  2430  	var recv *ast.FieldList
  2431  	if p.tok == token.LPAREN {
  2432  		recv = p.parseParameters(scope, false)
  2433  	}
  2434  
  2435  	ident := p.parseIdent()
  2436  
  2437  	params, results := p.parseSignature(scope)
  2438  
  2439  	var body *ast.BlockStmt
  2440  	if p.tok == token.LBRACE {
  2441  		body = p.parseBody(scope)
  2442  	}
  2443  	p.expectSemi()
  2444  
  2445  	decl := &ast.FuncDecl{
  2446  		Doc:  doc,
  2447  		Recv: recv,
  2448  		Name: ident,
  2449  		Type: &ast.FuncType{
  2450  			Func:    pos,
  2451  			Params:  params,
  2452  			Results: results,
  2453  		},
  2454  		Body: body,
  2455  	}
  2456  	if recv == nil {
  2457  		// Go spec: The scope of an identifier denoting a constant, type,
  2458  		// variable, or function (but not method) declared at top level
  2459  		// (outside any function) is the package block.
  2460  		//
  2461  		// init() functions cannot be referred to and there may
  2462  		// be more than one - don't put them in the pkgScope
  2463  		if ident.Name != "init" {
  2464  			p.declare(decl, nil, p.pkgScope, ast.Fun, ident)
  2465  		}
  2466  	}
  2467  
  2468  	return decl
  2469  }
  2470  
  2471  func (p *parser) parseDecl(sync map[token.Token]bool) ast.Decl {
  2472  	if p.trace {
  2473  		defer un(trace(p, "Declaration"))
  2474  	}
  2475  
  2476  	var f parseSpecFunction
  2477  	switch p.tok {
  2478  	case token.CONST, token.VAR:
  2479  		f = p.parseValueSpec
  2480  
  2481  	case token.TYPE:
  2482  		f = p.parseTypeSpec
  2483  
  2484  	case token.FUNC:
  2485  		return p.parseFuncDecl()
  2486  
  2487  	default:
  2488  		pos := p.pos
  2489  		p.errorExpected(pos, "declaration")
  2490  		p.advance(sync)
  2491  		return &ast.BadDecl{From: pos, To: p.pos}
  2492  	}
  2493  
  2494  	return p.parseGenDecl(p.tok, f)
  2495  }
  2496  
  2497  // ----------------------------------------------------------------------------
  2498  // Source files
  2499  
  2500  func (p *parser) parseFile() *ast.File {
  2501  	if p.trace {
  2502  		defer un(trace(p, "File"))
  2503  	}
  2504  
  2505  	// Don't bother parsing the rest if we had errors scanning the first token.
  2506  	// Likely not a Go source file at all.
  2507  	if p.errors.Len() != 0 {
  2508  		return nil
  2509  	}
  2510  
  2511  	// package clause
  2512  	doc := p.leadComment
  2513  	pos := p.expect(token.PACKAGE)
  2514  	// Go spec: The package clause is not a declaration;
  2515  	// the package name does not appear in any scope.
  2516  	ident := p.parseIdent()
  2517  	if ident.Name == "_" && p.mode&DeclarationErrors != 0 {
  2518  		p.error(p.pos, "invalid package name _")
  2519  	}
  2520  	p.expectSemi()
  2521  
  2522  	// Don't bother parsing the rest if we had errors parsing the package clause.
  2523  	// Likely not a Go source file at all.
  2524  	if p.errors.Len() != 0 {
  2525  		return nil
  2526  	}
  2527  
  2528  	p.openScope()
  2529  	p.pkgScope = p.topScope
  2530  	var decls []ast.Decl
  2531  	if p.mode&PackageClauseOnly == 0 {
  2532  		// import decls
  2533  		for p.tok == token.IMPORT {
  2534  			decls = append(decls, p.parseGenDecl(token.IMPORT, p.parseImportSpec))
  2535  		}
  2536  
  2537  		if p.mode&ImportsOnly == 0 {
  2538  			// rest of package body
  2539  			for p.tok != token.EOF {
  2540  				decls = append(decls, p.parseDecl(declStart))
  2541  			}
  2542  		}
  2543  	}
  2544  	p.closeScope()
  2545  	assert(p.topScope == nil, "unbalanced scopes")
  2546  	assert(p.labelScope == nil, "unbalanced label scopes")
  2547  
  2548  	// resolve global identifiers within the same file
  2549  	i := 0
  2550  	for _, ident := range p.unresolved {
  2551  		// i <= index for current ident
  2552  		assert(ident.Obj == unresolved, "object already resolved")
  2553  		ident.Obj = p.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
  2554  		if ident.Obj == nil {
  2555  			p.unresolved[i] = ident
  2556  			i++
  2557  		}
  2558  	}
  2559  
  2560  	return &ast.File{
  2561  		Doc:        doc,
  2562  		Package:    pos,
  2563  		Name:       ident,
  2564  		Decls:      decls,
  2565  		Scope:      p.pkgScope,
  2566  		Imports:    p.imports,
  2567  		Unresolved: p.unresolved[0:i],
  2568  		Comments:   p.comments,
  2569  	}
  2570  }
  2571  

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