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

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