decl.go

     1  // Copyright 2014 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 types2
     6  
     7  import (
     8  	"cmd/compile/internal/syntax"
     9  	"fmt"
    10  	"go/constant"
    11  	. "internal/types/errors"
    12  )
    13  
    14  func (err *error_) recordAltDecl(obj Object) {
    15  	if pos := obj.Pos(); pos.IsKnown() {
    16  		// We use "other" rather than "previous" here because
    17  		// the first declaration seen may not be textually
    18  		// earlier in the source.
    19  		err.errorf(pos, "other declaration of %s", obj.Name())
    20  	}
    21  }
    22  
    23  func (check *Checker) declare(scope *Scope, id *syntax.Name, obj Object, pos syntax.Pos) {
    24  	// spec: "The blank identifier, represented by the underscore
    25  	// character _, may be used in a declaration like any other
    26  	// identifier but the declaration does not introduce a new
    27  	// binding."
    28  	if obj.Name() != "_" {
    29  		if alt := scope.Insert(obj); alt != nil {
    30  			var err error_
    31  			err.code = DuplicateDecl
    32  			err.errorf(obj, "%s redeclared in this block", obj.Name())
    33  			err.recordAltDecl(alt)
    34  			check.report(&err)
    35  			return
    36  		}
    37  		obj.setScopePos(pos)
    38  	}
    39  	if id != nil {
    40  		check.recordDef(id, obj)
    41  	}
    42  }
    43  
    44  // pathString returns a string of the form a->b-> ... ->g for a path [a, b, ... g].
    45  func pathString(path []Object) string {
    46  	var s string
    47  	for i, p := range path {
    48  		if i > 0 {
    49  			s += "->"
    50  		}
    51  		s += p.Name()
    52  	}
    53  	return s
    54  }
    55  
    56  // objDecl type-checks the declaration of obj in its respective (file) environment.
    57  // For the meaning of def, see Checker.definedType, in typexpr.go.
    58  func (check *Checker) objDecl(obj Object, def *TypeName) {
    59  	if check.conf.Trace && obj.Type() == nil {
    60  		if check.indent == 0 {
    61  			fmt.Println() // empty line between top-level objects for readability
    62  		}
    63  		check.trace(obj.Pos(), "-- checking %s (%s, objPath = %s)", obj, obj.color(), pathString(check.objPath))
    64  		check.indent++
    65  		defer func() {
    66  			check.indent--
    67  			check.trace(obj.Pos(), "=> %s (%s)", obj, obj.color())
    68  		}()
    69  	}
    70  
    71  	// Checking the declaration of obj means inferring its type
    72  	// (and possibly its value, for constants).
    73  	// An object's type (and thus the object) may be in one of
    74  	// three states which are expressed by colors:
    75  	//
    76  	// - an object whose type is not yet known is painted white (initial color)
    77  	// - an object whose type is in the process of being inferred is painted grey
    78  	// - an object whose type is fully inferred is painted black
    79  	//
    80  	// During type inference, an object's color changes from white to grey
    81  	// to black (pre-declared objects are painted black from the start).
    82  	// A black object (i.e., its type) can only depend on (refer to) other black
    83  	// ones. White and grey objects may depend on white and black objects.
    84  	// A dependency on a grey object indicates a cycle which may or may not be
    85  	// valid.
    86  	//
    87  	// When objects turn grey, they are pushed on the object path (a stack);
    88  	// they are popped again when they turn black. Thus, if a grey object (a
    89  	// cycle) is encountered, it is on the object path, and all the objects
    90  	// it depends on are the remaining objects on that path. Color encoding
    91  	// is such that the color value of a grey object indicates the index of
    92  	// that object in the object path.
    93  
    94  	// During type-checking, white objects may be assigned a type without
    95  	// traversing through objDecl; e.g., when initializing constants and
    96  	// variables. Update the colors of those objects here (rather than
    97  	// everywhere where we set the type) to satisfy the color invariants.
    98  	if obj.color() == white && obj.Type() != nil {
    99  		obj.setColor(black)
   100  		return
   101  	}
   102  
   103  	switch obj.color() {
   104  	case white:
   105  		assert(obj.Type() == nil)
   106  		// All color values other than white and black are considered grey.
   107  		// Because black and white are < grey, all values >= grey are grey.
   108  		// Use those values to encode the object's index into the object path.
   109  		obj.setColor(grey + color(check.push(obj)))
   110  		defer func() {
   111  			check.pop().setColor(black)
   112  		}()
   113  
   114  	case black:
   115  		assert(obj.Type() != nil)
   116  		return
   117  
   118  	default:
   119  		// Color values other than white or black are considered grey.
   120  		fallthrough
   121  
   122  	case grey:
   123  		// We have a (possibly invalid) cycle.
   124  		// In the existing code, this is marked by a non-nil type
   125  		// for the object except for constants and variables whose
   126  		// type may be non-nil (known), or nil if it depends on the
   127  		// not-yet known initialization value.
   128  		// In the former case, set the type to Typ[Invalid] because
   129  		// we have an initialization cycle. The cycle error will be
   130  		// reported later, when determining initialization order.
   131  		// TODO(gri) Report cycle here and simplify initialization
   132  		// order code.
   133  		switch obj := obj.(type) {
   134  		case *Const:
   135  			if !check.validCycle(obj) || obj.typ == nil {
   136  				obj.typ = Typ[Invalid]
   137  			}
   138  
   139  		case *Var:
   140  			if !check.validCycle(obj) || obj.typ == nil {
   141  				obj.typ = Typ[Invalid]
   142  			}
   143  
   144  		case *TypeName:
   145  			if !check.validCycle(obj) {
   146  				// break cycle
   147  				// (without this, calling underlying()
   148  				// below may lead to an endless loop
   149  				// if we have a cycle for a defined
   150  				// (*Named) type)
   151  				obj.typ = Typ[Invalid]
   152  			}
   153  
   154  		case *Func:
   155  			if !check.validCycle(obj) {
   156  				// Don't set obj.typ to Typ[Invalid] here
   157  				// because plenty of code type-asserts that
   158  				// functions have a *Signature type. Grey
   159  				// functions have their type set to an empty
   160  				// signature which makes it impossible to
   161  				// initialize a variable with the function.
   162  			}
   163  
   164  		default:
   165  			unreachable()
   166  		}
   167  		assert(obj.Type() != nil)
   168  		return
   169  	}
   170  
   171  	d := check.objMap[obj]
   172  	if d == nil {
   173  		check.dump("%v: %s should have been declared", obj.Pos(), obj)
   174  		unreachable()
   175  	}
   176  
   177  	// save/restore current environment and set up object environment
   178  	defer func(env environment) {
   179  		check.environment = env
   180  	}(check.environment)
   181  	check.environment = environment{
   182  		scope: d.file,
   183  	}
   184  
   185  	// Const and var declarations must not have initialization
   186  	// cycles. We track them by remembering the current declaration
   187  	// in check.decl. Initialization expressions depending on other
   188  	// consts, vars, or functions, add dependencies to the current
   189  	// check.decl.
   190  	switch obj := obj.(type) {
   191  	case *Const:
   192  		check.decl = d // new package-level const decl
   193  		check.constDecl(obj, d.vtyp, d.init, d.inherited)
   194  	case *Var:
   195  		check.decl = d // new package-level var decl
   196  		check.varDecl(obj, d.lhs, d.vtyp, d.init)
   197  	case *TypeName:
   198  		// invalid recursive types are detected via path
   199  		check.typeDecl(obj, d.tdecl, def)
   200  		check.collectMethods(obj) // methods can only be added to top-level types
   201  	case *Func:
   202  		// functions may be recursive - no need to track dependencies
   203  		check.funcDecl(obj, d)
   204  	default:
   205  		unreachable()
   206  	}
   207  }
   208  
   209  // validCycle reports whether the cycle starting with obj is valid and
   210  // reports an error if it is not.
   211  func (check *Checker) validCycle(obj Object) (valid bool) {
   212  	// The object map contains the package scope objects and the non-interface methods.
   213  	if debug {
   214  		info := check.objMap[obj]
   215  		inObjMap := info != nil && (info.fdecl == nil || info.fdecl.Recv == nil) // exclude methods
   216  		isPkgObj := obj.Parent() == check.pkg.scope
   217  		if isPkgObj != inObjMap {
   218  			check.dump("%v: inconsistent object map for %s (isPkgObj = %v, inObjMap = %v)", obj.Pos(), obj, isPkgObj, inObjMap)
   219  			unreachable()
   220  		}
   221  	}
   222  
   223  	// Count cycle objects.
   224  	assert(obj.color() >= grey)
   225  	start := obj.color() - grey // index of obj in objPath
   226  	cycle := check.objPath[start:]
   227  	tparCycle := false // if set, the cycle is through a type parameter list
   228  	nval := 0          // number of (constant or variable) values in the cycle; valid if !generic
   229  	ndef := 0          // number of type definitions in the cycle; valid if !generic
   230  loop:
   231  	for _, obj := range cycle {
   232  		switch obj := obj.(type) {
   233  		case *Const, *Var:
   234  			nval++
   235  		case *TypeName:
   236  			// If we reach a generic type that is part of a cycle
   237  			// and we are in a type parameter list, we have a cycle
   238  			// through a type parameter list, which is invalid.
   239  			if check.inTParamList && isGeneric(obj.typ) {
   240  				tparCycle = true
   241  				break loop
   242  			}
   243  
   244  			// Determine if the type name is an alias or not. For
   245  			// package-level objects, use the object map which
   246  			// provides syntactic information (which doesn't rely
   247  			// on the order in which the objects are set up). For
   248  			// local objects, we can rely on the order, so use
   249  			// the object's predicate.
   250  			// TODO(gri) It would be less fragile to always access
   251  			// the syntactic information. We should consider storing
   252  			// this information explicitly in the object.
   253  			var alias bool
   254  			if check.enableAlias {
   255  				alias = obj.IsAlias()
   256  			} else {
   257  				if d := check.objMap[obj]; d != nil {
   258  					alias = d.tdecl.Alias // package-level object
   259  				} else {
   260  					alias = obj.IsAlias() // function local object
   261  				}
   262  			}
   263  			if !alias {
   264  				ndef++
   265  			}
   266  		case *Func:
   267  			// ignored for now
   268  		default:
   269  			unreachable()
   270  		}
   271  	}
   272  
   273  	if check.conf.Trace {
   274  		check.trace(obj.Pos(), "## cycle detected: objPath = %s->%s (len = %d)", pathString(cycle), obj.Name(), len(cycle))
   275  		if tparCycle {
   276  			check.trace(obj.Pos(), "## cycle contains: generic type in a type parameter list")
   277  		} else {
   278  			check.trace(obj.Pos(), "## cycle contains: %d values, %d type definitions", nval, ndef)
   279  		}
   280  		defer func() {
   281  			if valid {
   282  				check.trace(obj.Pos(), "=> cycle is valid")
   283  			} else {
   284  				check.trace(obj.Pos(), "=> error: cycle is invalid")
   285  			}
   286  		}()
   287  	}
   288  
   289  	if !tparCycle {
   290  		// A cycle involving only constants and variables is invalid but we
   291  		// ignore them here because they are reported via the initialization
   292  		// cycle check.
   293  		if nval == len(cycle) {
   294  			return true
   295  		}
   296  
   297  		// A cycle involving only types (and possibly functions) must have at least
   298  		// one type definition to be permitted: If there is no type definition, we
   299  		// have a sequence of alias type names which will expand ad infinitum.
   300  		if nval == 0 && ndef > 0 {
   301  			return true
   302  		}
   303  	}
   304  
   305  	check.cycleError(cycle)
   306  	return false
   307  }
   308  
   309  // cycleError reports a declaration cycle starting with
   310  // the object in cycle that is "first" in the source.
   311  func (check *Checker) cycleError(cycle []Object) {
   312  	// name returns the (possibly qualified) object name.
   313  	// This is needed because with generic types, cycles
   314  	// may refer to imported types. See go.dev/issue/50788.
   315  	// TODO(gri) This functionality is used elsewhere. Factor it out.
   316  	name := func(obj Object) string {
   317  		return packagePrefix(obj.Pkg(), check.qualifier) + obj.Name()
   318  	}
   319  
   320  	// TODO(gri) Should we start with the last (rather than the first) object in the cycle
   321  	//           since that is the earliest point in the source where we start seeing the
   322  	//           cycle? That would be more consistent with other error messages.
   323  	i := firstInSrc(cycle)
   324  	obj := cycle[i]
   325  	objName := name(obj)
   326  	// If obj is a type alias, mark it as valid (not broken) in order to avoid follow-on errors.
   327  	tname, _ := obj.(*TypeName)
   328  	if tname != nil && tname.IsAlias() {
   329  		// If we use Alias nodes, it is initialized with Typ[Invalid].
   330  		// TODO(gri) Adjust this code if we initialize with nil.
   331  		if !check.enableAlias {
   332  			check.validAlias(tname, Typ[Invalid])
   333  		}
   334  	}
   335  
   336  	// report a more concise error for self references
   337  	if len(cycle) == 1 {
   338  		if tname != nil {
   339  			check.errorf(obj, InvalidDeclCycle, "invalid recursive type: %s refers to itself", objName)
   340  		} else {
   341  			check.errorf(obj, InvalidDeclCycle, "invalid cycle in declaration: %s refers to itself", objName)
   342  		}
   343  		return
   344  	}
   345  
   346  	var err error_
   347  	err.code = InvalidDeclCycle
   348  	if tname != nil {
   349  		err.errorf(obj, "invalid recursive type %s", objName)
   350  	} else {
   351  		err.errorf(obj, "invalid cycle in declaration of %s", objName)
   352  	}
   353  	for range cycle {
   354  		err.errorf(obj, "%s refers to", objName)
   355  		i++
   356  		if i >= len(cycle) {
   357  			i = 0
   358  		}
   359  		obj = cycle[i]
   360  		objName = name(obj)
   361  	}
   362  	err.errorf(obj, "%s", objName)
   363  	check.report(&err)
   364  }
   365  
   366  // firstInSrc reports the index of the object with the "smallest"
   367  // source position in path. path must not be empty.
   368  func firstInSrc(path []Object) int {
   369  	fst, pos := 0, path[0].Pos()
   370  	for i, t := range path[1:] {
   371  		if cmpPos(t.Pos(), pos) < 0 {
   372  			fst, pos = i+1, t.Pos()
   373  		}
   374  	}
   375  	return fst
   376  }
   377  
   378  func (check *Checker) constDecl(obj *Const, typ, init syntax.Expr, inherited bool) {
   379  	assert(obj.typ == nil)
   380  
   381  	// use the correct value of iota and errpos
   382  	defer func(iota constant.Value, errpos syntax.Pos) {
   383  		check.iota = iota
   384  		check.errpos = errpos
   385  	}(check.iota, check.errpos)
   386  	check.iota = obj.val
   387  	check.errpos = nopos
   388  
   389  	// provide valid constant value under all circumstances
   390  	obj.val = constant.MakeUnknown()
   391  
   392  	// determine type, if any
   393  	if typ != nil {
   394  		t := check.typ(typ)
   395  		if !isConstType(t) {
   396  			// don't report an error if the type is an invalid C (defined) type
   397  			// (go.dev/issue/22090)
   398  			if isValid(under(t)) {
   399  				check.errorf(typ, InvalidConstType, "invalid constant type %s", t)
   400  			}
   401  			obj.typ = Typ[Invalid]
   402  			return
   403  		}
   404  		obj.typ = t
   405  	}
   406  
   407  	// check initialization
   408  	var x operand
   409  	if init != nil {
   410  		if inherited {
   411  			// The initialization expression is inherited from a previous
   412  			// constant declaration, and (error) positions refer to that
   413  			// expression and not the current constant declaration. Use
   414  			// the constant identifier position for any errors during
   415  			// init expression evaluation since that is all we have
   416  			// (see issues go.dev/issue/42991, go.dev/issue/42992).
   417  			check.errpos = obj.pos
   418  		}
   419  		check.expr(nil, &x, init)
   420  	}
   421  	check.initConst(obj, &x)
   422  }
   423  
   424  func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.Expr) {
   425  	assert(obj.typ == nil)
   426  
   427  	// determine type, if any
   428  	if typ != nil {
   429  		obj.typ = check.varType(typ)
   430  		// We cannot spread the type to all lhs variables if there
   431  		// are more than one since that would mark them as checked
   432  		// (see Checker.objDecl) and the assignment of init exprs,
   433  		// if any, would not be checked.
   434  		//
   435  		// TODO(gri) If we have no init expr, we should distribute
   436  		// a given type otherwise we need to re-evalate the type
   437  		// expr for each lhs variable, leading to duplicate work.
   438  	}
   439  
   440  	// check initialization
   441  	if init == nil {
   442  		if typ == nil {
   443  			// error reported before by arityMatch
   444  			obj.typ = Typ[Invalid]
   445  		}
   446  		return
   447  	}
   448  
   449  	if lhs == nil || len(lhs) == 1 {
   450  		assert(lhs == nil || lhs[0] == obj)
   451  		var x operand
   452  		check.expr(newTarget(obj.typ, obj.name), &x, init)
   453  		check.initVar(obj, &x, "variable declaration")
   454  		return
   455  	}
   456  
   457  	if debug {
   458  		// obj must be one of lhs
   459  		found := false
   460  		for _, lhs := range lhs {
   461  			if obj == lhs {
   462  				found = true
   463  				break
   464  			}
   465  		}
   466  		if !found {
   467  			panic("inconsistent lhs")
   468  		}
   469  	}
   470  
   471  	// We have multiple variables on the lhs and one init expr.
   472  	// Make sure all variables have been given the same type if
   473  	// one was specified, otherwise they assume the type of the
   474  	// init expression values (was go.dev/issue/15755).
   475  	if typ != nil {
   476  		for _, lhs := range lhs {
   477  			lhs.typ = obj.typ
   478  		}
   479  	}
   480  
   481  	check.initVars(lhs, []syntax.Expr{init}, nil)
   482  }
   483  
   484  // isImportedConstraint reports whether typ is an imported type constraint.
   485  func (check *Checker) isImportedConstraint(typ Type) bool {
   486  	named := asNamed(typ)
   487  	if named == nil || named.obj.pkg == check.pkg || named.obj.pkg == nil {
   488  		return false
   489  	}
   490  	u, _ := named.under().(*Interface)
   491  	return u != nil && !u.IsMethodSet()
   492  }
   493  
   494  func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *TypeName) {
   495  	assert(obj.typ == nil)
   496  
   497  	var rhs Type
   498  	check.later(func() {
   499  		if t := asNamed(obj.typ); t != nil { // type may be invalid
   500  			check.validType(t)
   501  		}
   502  		// If typ is local, an error was already reported where typ is specified/defined.
   503  		_ = check.isImportedConstraint(rhs) && check.verifyVersionf(tdecl.Type, go1_18, "using type constraint %s", rhs)
   504  	}).describef(obj, "validType(%s)", obj.Name())
   505  
   506  	aliasDecl := tdecl.Alias
   507  	if aliasDecl && tdecl.TParamList != nil {
   508  		// The parser will ensure this but we may still get an invalid AST.
   509  		// Complain and continue as regular type definition.
   510  		check.error(tdecl, BadDecl, "generic type cannot be alias")
   511  		aliasDecl = false
   512  	}
   513  
   514  	// alias declaration
   515  	if aliasDecl {
   516  		check.verifyVersionf(tdecl, go1_9, "type aliases")
   517  		if check.enableAlias {
   518  			// TODO(gri) Should be able to use nil instead of Typ[Invalid] to mark
   519  			//           the alias as incomplete. Currently this causes problems
   520  			//           with certain cycles. Investigate.
   521  			alias := check.newAlias(obj, Typ[Invalid])
   522  			setDefType(def, alias)
   523  			rhs = check.definedType(tdecl.Type, obj)
   524  			assert(rhs != nil)
   525  			alias.fromRHS = rhs
   526  			Unalias(alias) // resolve alias.actual
   527  		} else {
   528  			check.brokenAlias(obj)
   529  			rhs = check.typ(tdecl.Type)
   530  			check.validAlias(obj, rhs)
   531  		}
   532  		return
   533  	}
   534  
   535  	// type definition or generic type declaration
   536  	named := check.newNamed(obj, nil, nil)
   537  	setDefType(def, named)
   538  
   539  	if tdecl.TParamList != nil {
   540  		check.openScope(tdecl, "type parameters")
   541  		defer check.closeScope()
   542  		check.collectTypeParams(&named.tparams, tdecl.TParamList)
   543  	}
   544  
   545  	// determine underlying type of named
   546  	rhs = check.definedType(tdecl.Type, obj)
   547  	assert(rhs != nil)
   548  	named.fromRHS = rhs
   549  
   550  	// If the underlying type was not set while type-checking the right-hand
   551  	// side, it is invalid and an error should have been reported elsewhere.
   552  	if named.underlying == nil {
   553  		named.underlying = Typ[Invalid]
   554  	}
   555  
   556  	// Disallow a lone type parameter as the RHS of a type declaration (go.dev/issue/45639).
   557  	// We don't need this restriction anymore if we make the underlying type of a type
   558  	// parameter its constraint interface: if the RHS is a lone type parameter, we will
   559  	// use its underlying type (like we do for any RHS in a type declaration), and its
   560  	// underlying type is an interface and the type declaration is well defined.
   561  	if isTypeParam(rhs) {
   562  		check.error(tdecl.Type, MisplacedTypeParam, "cannot use a type parameter as RHS in type declaration")
   563  		named.underlying = Typ[Invalid]
   564  	}
   565  }
   566  
   567  func (check *Checker) collectTypeParams(dst **TypeParamList, list []*syntax.Field) {
   568  	tparams := make([]*TypeParam, len(list))
   569  
   570  	// Declare type parameters up-front.
   571  	// The scope of type parameters starts at the beginning of the type parameter
   572  	// list (so we can have mutually recursive parameterized type bounds).
   573  	if len(list) > 0 {
   574  		scopePos := list[0].Pos()
   575  		for i, f := range list {
   576  			tparams[i] = check.declareTypeParam(f.Name, scopePos)
   577  		}
   578  	}
   579  
   580  	// Set the type parameters before collecting the type constraints because
   581  	// the parameterized type may be used by the constraints (go.dev/issue/47887).
   582  	// Example: type T[P T[P]] interface{}
   583  	*dst = bindTParams(tparams)
   584  
   585  	// Signal to cycle detection that we are in a type parameter list.
   586  	// We can only be inside one type parameter list at any given time:
   587  	// function closures may appear inside a type parameter list but they
   588  	// cannot be generic, and their bodies are processed in delayed and
   589  	// sequential fashion. Note that with each new declaration, we save
   590  	// the existing environment and restore it when done; thus inTParamList
   591  	// is true exactly only when we are in a specific type parameter list.
   592  	assert(!check.inTParamList)
   593  	check.inTParamList = true
   594  	defer func() {
   595  		check.inTParamList = false
   596  	}()
   597  
   598  	// Keep track of bounds for later validation.
   599  	var bound Type
   600  	for i, f := range list {
   601  		// Optimization: Re-use the previous type bound if it hasn't changed.
   602  		// This also preserves the grouped output of type parameter lists
   603  		// when printing type strings.
   604  		if i == 0 || f.Type != list[i-1].Type {
   605  			bound = check.bound(f.Type)
   606  			if isTypeParam(bound) {
   607  				// We may be able to allow this since it is now well-defined what
   608  				// the underlying type and thus type set of a type parameter is.
   609  				// But we may need some additional form of cycle detection within
   610  				// type parameter lists.
   611  				check.error(f.Type, MisplacedTypeParam, "cannot use a type parameter as constraint")
   612  				bound = Typ[Invalid]
   613  			}
   614  		}
   615  		tparams[i].bound = bound
   616  	}
   617  }
   618  
   619  func (check *Checker) bound(x syntax.Expr) Type {
   620  	// A type set literal of the form ~T and A|B may only appear as constraint;
   621  	// embed it in an implicit interface so that only interface type-checking
   622  	// needs to take care of such type expressions.
   623  	if op, _ := x.(*syntax.Operation); op != nil && (op.Op == syntax.Tilde || op.Op == syntax.Or) {
   624  		t := check.typ(&syntax.InterfaceType{MethodList: []*syntax.Field{{Type: x}}})
   625  		// mark t as implicit interface if all went well
   626  		if t, _ := t.(*Interface); t != nil {
   627  			t.implicit = true
   628  		}
   629  		return t
   630  	}
   631  	return check.typ(x)
   632  }
   633  
   634  func (check *Checker) declareTypeParam(name *syntax.Name, scopePos syntax.Pos) *TypeParam {
   635  	// Use Typ[Invalid] for the type constraint to ensure that a type
   636  	// is present even if the actual constraint has not been assigned
   637  	// yet.
   638  	// TODO(gri) Need to systematically review all uses of type parameter
   639  	//           constraints to make sure we don't rely on them if they
   640  	//           are not properly set yet.
   641  	tname := NewTypeName(name.Pos(), check.pkg, name.Value, nil)
   642  	tpar := check.newTypeParam(tname, Typ[Invalid]) // assigns type to tname as a side-effect
   643  	check.declare(check.scope, name, tname, scopePos)
   644  	return tpar
   645  }
   646  
   647  func (check *Checker) collectMethods(obj *TypeName) {
   648  	// get associated methods
   649  	// (Checker.collectObjects only collects methods with non-blank names;
   650  	// Checker.resolveBaseTypeName ensures that obj is not an alias name
   651  	// if it has attached methods.)
   652  	methods := check.methods[obj]
   653  	if methods == nil {
   654  		return
   655  	}
   656  	delete(check.methods, obj)
   657  	assert(!check.objMap[obj].tdecl.Alias) // don't use TypeName.IsAlias (requires fully set up object)
   658  
   659  	// use an objset to check for name conflicts
   660  	var mset objset
   661  
   662  	// spec: "If the base type is a struct type, the non-blank method
   663  	// and field names must be distinct."
   664  	base := asNamed(obj.typ) // shouldn't fail but be conservative
   665  	if base != nil {
   666  		assert(base.TypeArgs().Len() == 0) // collectMethods should not be called on an instantiated type
   667  
   668  		// See go.dev/issue/52529: we must delay the expansion of underlying here, as
   669  		// base may not be fully set-up.
   670  		check.later(func() {
   671  			check.checkFieldUniqueness(base)
   672  		}).describef(obj, "verifying field uniqueness for %v", base)
   673  
   674  		// Checker.Files may be called multiple times; additional package files
   675  		// may add methods to already type-checked types. Add pre-existing methods
   676  		// so that we can detect redeclarations.
   677  		for i := 0; i < base.NumMethods(); i++ {
   678  			m := base.Method(i)
   679  			assert(m.name != "_")
   680  			assert(mset.insert(m) == nil)
   681  		}
   682  	}
   683  
   684  	// add valid methods
   685  	for _, m := range methods {
   686  		// spec: "For a base type, the non-blank names of methods bound
   687  		// to it must be unique."
   688  		assert(m.name != "_")
   689  		if alt := mset.insert(m); alt != nil {
   690  			if alt.Pos().IsKnown() {
   691  				check.errorf(m.pos, DuplicateMethod, "method %s.%s already declared at %s", obj.Name(), m.name, alt.Pos())
   692  			} else {
   693  				check.errorf(m.pos, DuplicateMethod, "method %s.%s already declared", obj.Name(), m.name)
   694  			}
   695  			continue
   696  		}
   697  
   698  		if base != nil {
   699  			base.AddMethod(m)
   700  		}
   701  	}
   702  }
   703  
   704  func (check *Checker) checkFieldUniqueness(base *Named) {
   705  	if t, _ := base.under().(*Struct); t != nil {
   706  		var mset objset
   707  		for i := 0; i < base.NumMethods(); i++ {
   708  			m := base.Method(i)
   709  			assert(m.name != "_")
   710  			assert(mset.insert(m) == nil)
   711  		}
   712  
   713  		// Check that any non-blank field names of base are distinct from its
   714  		// method names.
   715  		for _, fld := range t.fields {
   716  			if fld.name != "_" {
   717  				if alt := mset.insert(fld); alt != nil {
   718  					// Struct fields should already be unique, so we should only
   719  					// encounter an alternate via collision with a method name.
   720  					_ = alt.(*Func)
   721  
   722  					// For historical consistency, we report the primary error on the
   723  					// method, and the alt decl on the field.
   724  					var err error_
   725  					err.code = DuplicateFieldAndMethod
   726  					err.errorf(alt, "field and method with the same name %s", fld.name)
   727  					err.recordAltDecl(fld)
   728  					check.report(&err)
   729  				}
   730  			}
   731  		}
   732  	}
   733  }
   734  
   735  func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
   736  	assert(obj.typ == nil)
   737  
   738  	// func declarations cannot use iota
   739  	assert(check.iota == nil)
   740  
   741  	sig := new(Signature)
   742  	obj.typ = sig // guard against cycles
   743  
   744  	// Avoid cycle error when referring to method while type-checking the signature.
   745  	// This avoids a nuisance in the best case (non-parameterized receiver type) and
   746  	// since the method is not a type, we get an error. If we have a parameterized
   747  	// receiver type, instantiating the receiver type leads to the instantiation of
   748  	// its methods, and we don't want a cycle error in that case.
   749  	// TODO(gri) review if this is correct and/or whether we still need this?
   750  	saved := obj.color_
   751  	obj.color_ = black
   752  	fdecl := decl.fdecl
   753  	check.funcType(sig, fdecl.Recv, fdecl.TParamList, fdecl.Type)
   754  	obj.color_ = saved
   755  
   756  	// Set the scope's extent to the complete "func (...) { ... }"
   757  	// so that Scope.Innermost works correctly.
   758  	sig.scope.pos = fdecl.Pos()
   759  	sig.scope.end = syntax.EndPos(fdecl)
   760  
   761  	if len(fdecl.TParamList) > 0 && fdecl.Body == nil {
   762  		check.softErrorf(fdecl, BadDecl, "generic function is missing function body")
   763  	}
   764  
   765  	// function body must be type-checked after global declarations
   766  	// (functions implemented elsewhere have no body)
   767  	if !check.conf.IgnoreFuncBodies && fdecl.Body != nil {
   768  		check.later(func() {
   769  			check.funcBody(decl, obj.name, sig, fdecl.Body, nil)
   770  		}).describef(obj, "func %s", obj.name)
   771  	}
   772  }
   773  
   774  func (check *Checker) declStmt(list []syntax.Decl) {
   775  	pkg := check.pkg
   776  
   777  	first := -1                // index of first ConstDecl in the current group, or -1
   778  	var last *syntax.ConstDecl // last ConstDecl with init expressions, or nil
   779  	for index, decl := range list {
   780  		if _, ok := decl.(*syntax.ConstDecl); !ok {
   781  			first = -1 // we're not in a constant declaration
   782  		}
   783  
   784  		switch s := decl.(type) {
   785  		case *syntax.ConstDecl:
   786  			top := len(check.delayed)
   787  
   788  			// iota is the index of the current constDecl within the group
   789  			if first < 0 || s.Group == nil || list[index-1].(*syntax.ConstDecl).Group != s.Group {
   790  				first = index
   791  				last = nil
   792  			}
   793  			iota := constant.MakeInt64(int64(index - first))
   794  
   795  			// determine which initialization expressions to use
   796  			inherited := true
   797  			switch {
   798  			case s.Type != nil || s.Values != nil:
   799  				last = s
   800  				inherited = false
   801  			case last == nil:
   802  				last = new(syntax.ConstDecl) // make sure last exists
   803  				inherited = false
   804  			}
   805  
   806  			// declare all constants
   807  			lhs := make([]*Const, len(s.NameList))
   808  			values := syntax.UnpackListExpr(last.Values)
   809  			for i, name := range s.NameList {
   810  				obj := NewConst(name.Pos(), pkg, name.Value, nil, iota)
   811  				lhs[i] = obj
   812  
   813  				var init syntax.Expr
   814  				if i < len(values) {
   815  					init = values[i]
   816  				}
   817  
   818  				check.constDecl(obj, last.Type, init, inherited)
   819  			}
   820  
   821  			// Constants must always have init values.
   822  			check.arity(s.Pos(), s.NameList, values, true, inherited)
   823  
   824  			// process function literals in init expressions before scope changes
   825  			check.processDelayed(top)
   826  
   827  			// spec: "The scope of a constant or variable identifier declared
   828  			// inside a function begins at the end of the ConstSpec or VarSpec
   829  			// (ShortVarDecl for short variable declarations) and ends at the
   830  			// end of the innermost containing block."
   831  			scopePos := syntax.EndPos(s)
   832  			for i, name := range s.NameList {
   833  				check.declare(check.scope, name, lhs[i], scopePos)
   834  			}
   835  
   836  		case *syntax.VarDecl:
   837  			top := len(check.delayed)
   838  
   839  			lhs0 := make([]*Var, len(s.NameList))
   840  			for i, name := range s.NameList {
   841  				lhs0[i] = NewVar(name.Pos(), pkg, name.Value, nil)
   842  			}
   843  
   844  			// initialize all variables
   845  			values := syntax.UnpackListExpr(s.Values)
   846  			for i, obj := range lhs0 {
   847  				var lhs []*Var
   848  				var init syntax.Expr
   849  				switch len(values) {
   850  				case len(s.NameList):
   851  					// lhs and rhs match
   852  					init = values[i]
   853  				case 1:
   854  					// rhs is expected to be a multi-valued expression
   855  					lhs = lhs0
   856  					init = values[0]
   857  				default:
   858  					if i < len(values) {
   859  						init = values[i]
   860  					}
   861  				}
   862  				check.varDecl(obj, lhs, s.Type, init)
   863  				if len(values) == 1 {
   864  					// If we have a single lhs variable we are done either way.
   865  					// If we have a single rhs expression, it must be a multi-
   866  					// valued expression, in which case handling the first lhs
   867  					// variable will cause all lhs variables to have a type
   868  					// assigned, and we are done as well.
   869  					if debug {
   870  						for _, obj := range lhs0 {
   871  							assert(obj.typ != nil)
   872  						}
   873  					}
   874  					break
   875  				}
   876  			}
   877  
   878  			// If we have no type, we must have values.
   879  			if s.Type == nil || values != nil {
   880  				check.arity(s.Pos(), s.NameList, values, false, false)
   881  			}
   882  
   883  			// process function literals in init expressions before scope changes
   884  			check.processDelayed(top)
   885  
   886  			// declare all variables
   887  			// (only at this point are the variable scopes (parents) set)
   888  			scopePos := syntax.EndPos(s) // see constant declarations
   889  			for i, name := range s.NameList {
   890  				// see constant declarations
   891  				check.declare(check.scope, name, lhs0[i], scopePos)
   892  			}
   893  
   894  		case *syntax.TypeDecl:
   895  			obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Value, nil)
   896  			// spec: "The scope of a type identifier declared inside a function
   897  			// begins at the identifier in the TypeSpec and ends at the end of
   898  			// the innermost containing block."
   899  			scopePos := s.Name.Pos()
   900  			check.declare(check.scope, s.Name, obj, scopePos)
   901  			// mark and unmark type before calling typeDecl; its type is still nil (see Checker.objDecl)
   902  			obj.setColor(grey + color(check.push(obj)))
   903  			check.typeDecl(obj, s, nil)
   904  			check.pop().setColor(black)
   905  
   906  		default:
   907  			check.errorf(s, InvalidSyntaxTree, "unknown syntax.Decl node %T", s)
   908  		}
   909  	}
   910  }
   911
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