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  )
    12  
    13  func (err *error_) recordAltDecl(obj Object) {
    14  	if pos := obj.Pos(); pos.IsKnown() {
    15  		// We use "other" rather than "previous" here because
    16  		// the first declaration seen may not be textually
    17  		// earlier in the source.
    18  		err.errorf(pos, "other declaration of %s", obj.Name())
    19  	}
    20  }
    21  
    22  func (check *Checker) declare(scope *Scope, id *syntax.Name, obj Object, pos syntax.Pos) {
    23  	// spec: "The blank identifier, represented by the underscore
    24  	// character _, may be used in a declaration like any other
    25  	// identifier but the declaration does not introduce a new
    26  	// binding."
    27  	if obj.Name() != "_" {
    28  		if alt := scope.Insert(obj); alt != nil {
    29  			var err error_
    30  			err.errorf(obj, "%s redeclared in this block", obj.Name())
    31  			err.recordAltDecl(alt)
    32  			check.report(&err)
    33  			return
    34  		}
    35  		obj.setScopePos(pos)
    36  	}
    37  	if id != nil {
    38  		check.recordDef(id, obj)
    39  	}
    40  }
    41  
    42  // pathString returns a string of the form a->b-> ... ->g for a path [a, b, ... g].
    43  func pathString(path []Object) string {
    44  	var s string
    45  	for i, p := range path {
    46  		if i > 0 {
    47  			s += "->"
    48  		}
    49  		s += p.Name()
    50  	}
    51  	return s
    52  }
    53  
    54  // objDecl type-checks the declaration of obj in its respective (file) environment.
    55  // For the meaning of def, see Checker.definedType, in typexpr.go.
    56  func (check *Checker) objDecl(obj Object, def *Named) {
    57  	if check.conf.Trace && obj.Type() == nil {
    58  		if check.indent == 0 {
    59  			fmt.Println() // empty line between top-level objects for readability
    60  		}
    61  		check.trace(obj.Pos(), "-- checking %s (%s, objPath = %s)", obj, obj.color(), pathString(check.objPath))
    62  		check.indent++
    63  		defer func() {
    64  			check.indent--
    65  			check.trace(obj.Pos(), "=> %s (%s)", obj, obj.color())
    66  		}()
    67  	}
    68  
    69  	// Checking the declaration of obj means inferring its type
    70  	// (and possibly its value, for constants).
    71  	// An object's type (and thus the object) may be in one of
    72  	// three states which are expressed by colors:
    73  	//
    74  	// - an object whose type is not yet known is painted white (initial color)
    75  	// - an object whose type is in the process of being inferred is painted grey
    76  	// - an object whose type is fully inferred is painted black
    77  	//
    78  	// During type inference, an object's color changes from white to grey
    79  	// to black (pre-declared objects are painted black from the start).
    80  	// A black object (i.e., its type) can only depend on (refer to) other black
    81  	// ones. White and grey objects may depend on white and black objects.
    82  	// A dependency on a grey object indicates a cycle which may or may not be
    83  	// valid.
    84  	//
    85  	// When objects turn grey, they are pushed on the object path (a stack);
    86  	// they are popped again when they turn black. Thus, if a grey object (a
    87  	// cycle) is encountered, it is on the object path, and all the objects
    88  	// it depends on are the remaining objects on that path. Color encoding
    89  	// is such that the color value of a grey object indicates the index of
    90  	// that object in the object path.
    91  
    92  	// During type-checking, white objects may be assigned a type without
    93  	// traversing through objDecl; e.g., when initializing constants and
    94  	// variables. Update the colors of those objects here (rather than
    95  	// everywhere where we set the type) to satisfy the color invariants.
    96  	if obj.color() == white && obj.Type() != nil {
    97  		obj.setColor(black)
    98  		return
    99  	}
   100  
   101  	switch obj.color() {
   102  	case white:
   103  		assert(obj.Type() == nil)
   104  		// All color values other than white and black are considered grey.
   105  		// Because black and white are < grey, all values >= grey are grey.
   106  		// Use those values to encode the object's index into the object path.
   107  		obj.setColor(grey + color(check.push(obj)))
   108  		defer func() {
   109  			check.pop().setColor(black)
   110  		}()
   111  
   112  	case black:
   113  		assert(obj.Type() != nil)
   114  		return
   115  
   116  	default:
   117  		// Color values other than white or black are considered grey.
   118  		fallthrough
   119  
   120  	case grey:
   121  		// We have a (possibly invalid) cycle.
   122  		// In the existing code, this is marked by a non-nil type
   123  		// for the object except for constants and variables whose
   124  		// type may be non-nil (known), or nil if it depends on the
   125  		// not-yet known initialization value.
   126  		// In the former case, set the type to Typ[Invalid] because
   127  		// we have an initialization cycle. The cycle error will be
   128  		// reported later, when determining initialization order.
   129  		// TODO(gri) Report cycle here and simplify initialization
   130  		// order code.
   131  		switch obj := obj.(type) {
   132  		case *Const:
   133  			if !check.validCycle(obj) || obj.typ == nil {
   134  				obj.typ = Typ[Invalid]
   135  			}
   136  
   137  		case *Var:
   138  			if !check.validCycle(obj) || obj.typ == nil {
   139  				obj.typ = Typ[Invalid]
   140  			}
   141  
   142  		case *TypeName:
   143  			if !check.validCycle(obj) {
   144  				// break cycle
   145  				// (without this, calling underlying()
   146  				// below may lead to an endless loop
   147  				// if we have a cycle for a defined
   148  				// (*Named) type)
   149  				obj.typ = Typ[Invalid]
   150  			}
   151  
   152  		case *Func:
   153  			if !check.validCycle(obj) {
   154  				// Don't set obj.typ to Typ[Invalid] here
   155  				// because plenty of code type-asserts that
   156  				// functions have a *Signature type. Grey
   157  				// functions have their type set to an empty
   158  				// signature which makes it impossible to
   159  				// initialize a variable with the function.
   160  			}
   161  
   162  		default:
   163  			unreachable()
   164  		}
   165  		assert(obj.Type() != nil)
   166  		return
   167  	}
   168  
   169  	d := check.objMap[obj]
   170  	if d == nil {
   171  		check.dump("%v: %s should have been declared", obj.Pos(), obj)
   172  		unreachable()
   173  	}
   174  
   175  	// save/restore current environment and set up object environment
   176  	defer func(env environment) {
   177  		check.environment = env
   178  	}(check.environment)
   179  	check.environment = environment{
   180  		scope: d.file,
   181  	}
   182  
   183  	// Const and var declarations must not have initialization
   184  	// cycles. We track them by remembering the current declaration
   185  	// in check.decl. Initialization expressions depending on other
   186  	// consts, vars, or functions, add dependencies to the current
   187  	// check.decl.
   188  	switch obj := obj.(type) {
   189  	case *Const:
   190  		check.decl = d // new package-level const decl
   191  		check.constDecl(obj, d.vtyp, d.init, d.inherited)
   192  	case *Var:
   193  		check.decl = d // new package-level var decl
   194  		check.varDecl(obj, d.lhs, d.vtyp, d.init)
   195  	case *TypeName:
   196  		// invalid recursive types are detected via path
   197  		check.typeDecl(obj, d.tdecl, def)
   198  		check.collectMethods(obj) // methods can only be added to top-level types
   199  	case *Func:
   200  		// functions may be recursive - no need to track dependencies
   201  		check.funcDecl(obj, d)
   202  	default:
   203  		unreachable()
   204  	}
   205  }
   206  
   207  // validCycle reports whether the cycle starting with obj is valid and
   208  // reports an error if it is not.
   209  func (check *Checker) validCycle(obj Object) (valid bool) {
   210  	// The object map contains the package scope objects and the non-interface methods.
   211  	if debug {
   212  		info := check.objMap[obj]
   213  		inObjMap := info != nil && (info.fdecl == nil || info.fdecl.Recv == nil) // exclude methods
   214  		isPkgObj := obj.Parent() == check.pkg.scope
   215  		if isPkgObj != inObjMap {
   216  			check.dump("%v: inconsistent object map for %s (isPkgObj = %v, inObjMap = %v)", obj.Pos(), obj, isPkgObj, inObjMap)
   217  			unreachable()
   218  		}
   219  	}
   220  
   221  	// Count cycle objects.
   222  	assert(obj.color() >= grey)
   223  	start := obj.color() - grey // index of obj in objPath
   224  	cycle := check.objPath[start:]
   225  	tparCycle := false // if set, the cycle is through a type parameter list
   226  	nval := 0          // number of (constant or variable) values in the cycle; valid if !generic
   227  	ndef := 0          // number of type definitions in the cycle; valid if !generic
   228  loop:
   229  	for _, obj := range cycle {
   230  		switch obj := obj.(type) {
   231  		case *Const, *Var:
   232  			nval++
   233  		case *TypeName:
   234  			// If we reach a generic type that is part of a cycle
   235  			// and we are in a type parameter list, we have a cycle
   236  			// through a type parameter list, which is invalid.
   237  			if check.inTParamList && isGeneric(obj.typ) {
   238  				tparCycle = true
   239  				break loop
   240  			}
   241  
   242  			// Determine if the type name is an alias or not. For
   243  			// package-level objects, use the object map which
   244  			// provides syntactic information (which doesn't rely
   245  			// on the order in which the objects are set up). For
   246  			// local objects, we can rely on the order, so use
   247  			// the object's predicate.
   248  			// TODO(gri) It would be less fragile to always access
   249  			// the syntactic information. We should consider storing
   250  			// this information explicitly in the object.
   251  			var alias bool
   252  			if d := check.objMap[obj]; d != nil {
   253  				alias = d.tdecl.Alias // package-level object
   254  			} else {
   255  				alias = obj.IsAlias() // function local object
   256  			}
   257  			if !alias {
   258  				ndef++
   259  			}
   260  		case *Func:
   261  			// ignored for now
   262  		default:
   263  			unreachable()
   264  		}
   265  	}
   266  
   267  	if check.conf.Trace {
   268  		check.trace(obj.Pos(), "## cycle detected: objPath = %s->%s (len = %d)", pathString(cycle), obj.Name(), len(cycle))
   269  		if tparCycle {
   270  			check.trace(obj.Pos(), "## cycle contains: generic type in a type parameter list")
   271  		} else {
   272  			check.trace(obj.Pos(), "## cycle contains: %d values, %d type definitions", nval, ndef)
   273  		}
   274  		defer func() {
   275  			if valid {
   276  				check.trace(obj.Pos(), "=> cycle is valid")
   277  			} else {
   278  				check.trace(obj.Pos(), "=> error: cycle is invalid")
   279  			}
   280  		}()
   281  	}
   282  
   283  	if !tparCycle {
   284  		// A cycle involving only constants and variables is invalid but we
   285  		// ignore them here because they are reported via the initialization
   286  		// cycle check.
   287  		if nval == len(cycle) {
   288  			return true
   289  		}
   290  
   291  		// A cycle involving only types (and possibly functions) must have at least
   292  		// one type definition to be permitted: If there is no type definition, we
   293  		// have a sequence of alias type names which will expand ad infinitum.
   294  		if nval == 0 && ndef > 0 {
   295  			return true
   296  		}
   297  	}
   298  
   299  	check.cycleError(cycle)
   300  	return false
   301  }
   302  
   303  // cycleError reports a declaration cycle starting with
   304  // the object in cycle that is "first" in the source.
   305  func (check *Checker) cycleError(cycle []Object) {
   306  	// TODO(gri) Should we start with the last (rather than the first) object in the cycle
   307  	//           since that is the earliest point in the source where we start seeing the
   308  	//           cycle? That would be more consistent with other error messages.
   309  	i := firstInSrc(cycle)
   310  	obj := cycle[i]
   311  	// If obj is a type alias, mark it as valid (not broken) in order to avoid follow-on errors.
   312  	tname, _ := obj.(*TypeName)
   313  	if tname != nil && tname.IsAlias() {
   314  		check.validAlias(tname, Typ[Invalid])
   315  	}
   316  	var err error_
   317  	if tname != nil && check.conf.CompilerErrorMessages {
   318  		err.errorf(obj, "invalid recursive type %s", obj.Name())
   319  	} else {
   320  		err.errorf(obj, "illegal cycle in declaration of %s", obj.Name())
   321  	}
   322  	for range cycle {
   323  		err.errorf(obj, "%s refers to", obj.Name())
   324  		i++
   325  		if i >= len(cycle) {
   326  			i = 0
   327  		}
   328  		obj = cycle[i]
   329  	}
   330  	err.errorf(obj, "%s", obj.Name())
   331  	check.report(&err)
   332  }
   333  
   334  // firstInSrc reports the index of the object with the "smallest"
   335  // source position in path. path must not be empty.
   336  func firstInSrc(path []Object) int {
   337  	fst, pos := 0, path[0].Pos()
   338  	for i, t := range path[1:] {
   339  		if t.Pos().Cmp(pos) < 0 {
   340  			fst, pos = i+1, t.Pos()
   341  		}
   342  	}
   343  	return fst
   344  }
   345  
   346  func (check *Checker) constDecl(obj *Const, typ, init syntax.Expr, inherited bool) {
   347  	assert(obj.typ == nil)
   348  
   349  	// use the correct value of iota and errpos
   350  	defer func(iota constant.Value, errpos syntax.Pos) {
   351  		check.iota = iota
   352  		check.errpos = errpos
   353  	}(check.iota, check.errpos)
   354  	check.iota = obj.val
   355  	check.errpos = nopos
   356  
   357  	// provide valid constant value under all circumstances
   358  	obj.val = constant.MakeUnknown()
   359  
   360  	// determine type, if any
   361  	if typ != nil {
   362  		t := check.typ(typ)
   363  		if !isConstType(t) {
   364  			// don't report an error if the type is an invalid C (defined) type
   365  			// (issue #22090)
   366  			if under(t) != Typ[Invalid] {
   367  				check.errorf(typ, "invalid constant type %s", t)
   368  			}
   369  			obj.typ = Typ[Invalid]
   370  			return
   371  		}
   372  		obj.typ = t
   373  	}
   374  
   375  	// check initialization
   376  	var x operand
   377  	if init != nil {
   378  		if inherited {
   379  			// The initialization expression is inherited from a previous
   380  			// constant declaration, and (error) positions refer to that
   381  			// expression and not the current constant declaration. Use
   382  			// the constant identifier position for any errors during
   383  			// init expression evaluation since that is all we have
   384  			// (see issues #42991, #42992).
   385  			check.errpos = obj.pos
   386  		}
   387  		check.expr(&x, init)
   388  	}
   389  	check.initConst(obj, &x)
   390  }
   391  
   392  func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.Expr) {
   393  	assert(obj.typ == nil)
   394  
   395  	// If we have undefined variable types due to errors,
   396  	// mark variables as used to avoid follow-on errors.
   397  	// Matches compiler behavior.
   398  	defer func() {
   399  		if obj.typ == Typ[Invalid] {
   400  			obj.used = true
   401  		}
   402  		for _, lhs := range lhs {
   403  			if lhs.typ == Typ[Invalid] {
   404  				lhs.used = true
   405  			}
   406  		}
   407  	}()
   408  
   409  	// determine type, if any
   410  	if typ != nil {
   411  		obj.typ = check.varType(typ)
   412  		// We cannot spread the type to all lhs variables if there
   413  		// are more than one since that would mark them as checked
   414  		// (see Checker.objDecl) and the assignment of init exprs,
   415  		// if any, would not be checked.
   416  		//
   417  		// TODO(gri) If we have no init expr, we should distribute
   418  		// a given type otherwise we need to re-evalate the type
   419  		// expr for each lhs variable, leading to duplicate work.
   420  	}
   421  
   422  	// check initialization
   423  	if init == nil {
   424  		if typ == nil {
   425  			// error reported before by arityMatch
   426  			obj.typ = Typ[Invalid]
   427  		}
   428  		return
   429  	}
   430  
   431  	if lhs == nil || len(lhs) == 1 {
   432  		assert(lhs == nil || lhs[0] == obj)
   433  		var x operand
   434  		check.expr(&x, init)
   435  		check.initVar(obj, &x, "variable declaration")
   436  		return
   437  	}
   438  
   439  	if debug {
   440  		// obj must be one of lhs
   441  		found := false
   442  		for _, lhs := range lhs {
   443  			if obj == lhs {
   444  				found = true
   445  				break
   446  			}
   447  		}
   448  		if !found {
   449  			panic("inconsistent lhs")
   450  		}
   451  	}
   452  
   453  	// We have multiple variables on the lhs and one init expr.
   454  	// Make sure all variables have been given the same type if
   455  	// one was specified, otherwise they assume the type of the
   456  	// init expression values (was issue #15755).
   457  	if typ != nil {
   458  		for _, lhs := range lhs {
   459  			lhs.typ = obj.typ
   460  		}
   461  	}
   462  
   463  	check.initVars(lhs, []syntax.Expr{init}, nil)
   464  }
   465  
   466  // isImportedConstraint reports whether typ is an imported type constraint.
   467  func (check *Checker) isImportedConstraint(typ Type) bool {
   468  	named, _ := typ.(*Named)
   469  	if named == nil || named.obj.pkg == check.pkg || named.obj.pkg == nil {
   470  		return false
   471  	}
   472  	u, _ := named.under().(*Interface)
   473  	return u != nil && !u.IsMethodSet()
   474  }
   475  
   476  func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named) {
   477  	assert(obj.typ == nil)
   478  
   479  	var rhs Type
   480  	check.later(func() {
   481  		if t, _ := obj.typ.(*Named); t != nil { // type may be invalid
   482  			check.validType(t)
   483  		}
   484  		// If typ is local, an error was already reported where typ is specified/defined.
   485  		if check.isImportedConstraint(rhs) && !check.allowVersion(check.pkg, 1, 18) {
   486  			check.versionErrorf(tdecl.Type, "go1.18", "using type constraint %s", rhs)
   487  		}
   488  	}).describef(obj, "validType(%s)", obj.Name())
   489  
   490  	alias := tdecl.Alias
   491  	if alias && tdecl.TParamList != nil {
   492  		// The parser will ensure this but we may still get an invalid AST.
   493  		// Complain and continue as regular type definition.
   494  		check.error(tdecl, "generic type cannot be alias")
   495  		alias = false
   496  	}
   497  
   498  	// alias declaration
   499  	if alias {
   500  		if !check.allowVersion(check.pkg, 1, 9) {
   501  			check.versionErrorf(tdecl, "go1.9", "type aliases")
   502  		}
   503  
   504  		check.brokenAlias(obj)
   505  		rhs = check.varType(tdecl.Type)
   506  		check.validAlias(obj, rhs)
   507  		return
   508  	}
   509  
   510  	// type definition or generic type declaration
   511  	named := check.newNamed(obj, nil, nil, nil, nil)
   512  	def.setUnderlying(named)
   513  
   514  	if tdecl.TParamList != nil {
   515  		check.openScope(tdecl, "type parameters")
   516  		defer check.closeScope()
   517  		check.collectTypeParams(&named.tparams, tdecl.TParamList)
   518  	}
   519  
   520  	// determine underlying type of named
   521  	rhs = check.definedType(tdecl.Type, named)
   522  	assert(rhs != nil)
   523  	named.fromRHS = rhs
   524  
   525  	// If the underlying was not set while type-checking the right-hand side, it
   526  	// is invalid and an error should have been reported elsewhere.
   527  	if named.underlying == nil {
   528  		named.underlying = Typ[Invalid]
   529  	}
   530  
   531  	// Disallow a lone type parameter as the RHS of a type declaration (issue #45639).
   532  	// We don't need this restriction anymore if we make the underlying type of a type
   533  	// parameter its constraint interface: if the RHS is a lone type parameter, we will
   534  	// use its underlying type (like we do for any RHS in a type declaration), and its
   535  	// underlying type is an interface and the type declaration is well defined.
   536  	if isTypeParam(rhs) {
   537  		check.error(tdecl.Type, "cannot use a type parameter as RHS in type declaration")
   538  		named.underlying = Typ[Invalid]
   539  	}
   540  }
   541  
   542  func (check *Checker) collectTypeParams(dst **TypeParamList, list []*syntax.Field) {
   543  	tparams := make([]*TypeParam, len(list))
   544  
   545  	// Declare type parameters up-front.
   546  	// The scope of type parameters starts at the beginning of the type parameter
   547  	// list (so we can have mutually recursive parameterized type bounds).
   548  	for i, f := range list {
   549  		tparams[i] = check.declareTypeParam(f.Name)
   550  	}
   551  
   552  	// Set the type parameters before collecting the type constraints because
   553  	// the parameterized type may be used by the constraints (issue #47887).
   554  	// Example: type T[P T[P]] interface{}
   555  	*dst = bindTParams(tparams)
   556  
   557  	// Signal to cycle detection that we are in a type parameter list.
   558  	// We can only be inside one type parameter list at any given time:
   559  	// function closures may appear inside a type parameter list but they
   560  	// cannot be generic, and their bodies are processed in delayed and
   561  	// sequential fashion. Note that with each new declaration, we save
   562  	// the existing environment and restore it when done; thus inTParamList
   563  	// is true exactly only when we are in a specific type parameter list.
   564  	assert(!check.inTParamList)
   565  	check.inTParamList = true
   566  	defer func() {
   567  		check.inTParamList = false
   568  	}()
   569  
   570  	// Keep track of bounds for later validation.
   571  	var bound Type
   572  	for i, f := range list {
   573  		// Optimization: Re-use the previous type bound if it hasn't changed.
   574  		// This also preserves the grouped output of type parameter lists
   575  		// when printing type strings.
   576  		if i == 0 || f.Type != list[i-1].Type {
   577  			bound = check.bound(f.Type)
   578  			if isTypeParam(bound) {
   579  				// We may be able to allow this since it is now well-defined what
   580  				// the underlying type and thus type set of a type parameter is.
   581  				// But we may need some additional form of cycle detection within
   582  				// type parameter lists.
   583  				check.error(f.Type, "cannot use a type parameter as constraint")
   584  				bound = Typ[Invalid]
   585  			}
   586  		}
   587  		tparams[i].bound = bound
   588  	}
   589  }
   590  
   591  func (check *Checker) bound(x syntax.Expr) Type {
   592  	// A type set literal of the form ~T and A|B may only appear as constraint;
   593  	// embed it in an implicit interface so that only interface type-checking
   594  	// needs to take care of such type expressions.
   595  	if op, _ := x.(*syntax.Operation); op != nil && (op.Op == syntax.Tilde || op.Op == syntax.Or) {
   596  		t := check.typ(&syntax.InterfaceType{MethodList: []*syntax.Field{{Type: x}}})
   597  		// mark t as implicit interface if all went well
   598  		if t, _ := t.(*Interface); t != nil {
   599  			t.implicit = true
   600  		}
   601  		return t
   602  	}
   603  	return check.typ(x)
   604  }
   605  
   606  func (check *Checker) declareTypeParam(name *syntax.Name) *TypeParam {
   607  	// Use Typ[Invalid] for the type constraint to ensure that a type
   608  	// is present even if the actual constraint has not been assigned
   609  	// yet.
   610  	// TODO(gri) Need to systematically review all uses of type parameter
   611  	//           constraints to make sure we don't rely on them if they
   612  	//           are not properly set yet.
   613  	tname := NewTypeName(name.Pos(), check.pkg, name.Value, nil)
   614  	tpar := check.newTypeParam(tname, Typ[Invalid])          // assigns type to tname as a side-effect
   615  	check.declare(check.scope, name, tname, check.scope.pos) // TODO(gri) check scope position
   616  	return tpar
   617  }
   618  
   619  func (check *Checker) collectMethods(obj *TypeName) {
   620  	// get associated methods
   621  	// (Checker.collectObjects only collects methods with non-blank names;
   622  	// Checker.resolveBaseTypeName ensures that obj is not an alias name
   623  	// if it has attached methods.)
   624  	methods := check.methods[obj]
   625  	if methods == nil {
   626  		return
   627  	}
   628  	delete(check.methods, obj)
   629  	assert(!check.objMap[obj].tdecl.Alias) // don't use TypeName.IsAlias (requires fully set up object)
   630  
   631  	// use an objset to check for name conflicts
   632  	var mset objset
   633  
   634  	// spec: "If the base type is a struct type, the non-blank method
   635  	// and field names must be distinct."
   636  	base, _ := obj.typ.(*Named) // shouldn't fail but be conservative
   637  	if base != nil {
   638  		assert(base.targs.Len() == 0) // collectMethods should not be called on an instantiated type
   639  
   640  		// See issue #52529: we must delay the expansion of underlying here, as
   641  		// base may not be fully set-up.
   642  		check.later(func() {
   643  			check.checkFieldUniqueness(base)
   644  		}).describef(obj, "verifying field uniqueness for %v", base)
   645  
   646  		// Checker.Files may be called multiple times; additional package files
   647  		// may add methods to already type-checked types. Add pre-existing methods
   648  		// so that we can detect redeclarations.
   649  		for i := 0; i < base.methods.Len(); i++ {
   650  			m := base.methods.At(i, nil)
   651  			assert(m.name != "_")
   652  			assert(mset.insert(m) == nil)
   653  		}
   654  	}
   655  
   656  	// add valid methods
   657  	for _, m := range methods {
   658  		// spec: "For a base type, the non-blank names of methods bound
   659  		// to it must be unique."
   660  		assert(m.name != "_")
   661  		if alt := mset.insert(m); alt != nil {
   662  			var err error_
   663  			if check.conf.CompilerErrorMessages {
   664  				err.errorf(m.pos, "%s.%s redeclared in this block", obj.Name(), m.name)
   665  			} else {
   666  				err.errorf(m.pos, "method %s already declared for %s", m.name, obj)
   667  			}
   668  			err.recordAltDecl(alt)
   669  			check.report(&err)
   670  			continue
   671  		}
   672  
   673  		if base != nil {
   674  			base.resolve(nil) // TODO(mdempsky): Probably unnecessary.
   675  			base.AddMethod(m)
   676  		}
   677  	}
   678  }
   679  
   680  func (check *Checker) checkFieldUniqueness(base *Named) {
   681  	if t, _ := base.under().(*Struct); t != nil {
   682  		var mset objset
   683  		for i := 0; i < base.methods.Len(); i++ {
   684  			m := base.methods.At(i, nil)
   685  			assert(m.name != "_")
   686  			assert(mset.insert(m) == nil)
   687  		}
   688  
   689  		// Check that any non-blank field names of base are distinct from its
   690  		// method names.
   691  		for _, fld := range t.fields {
   692  			if fld.name != "_" {
   693  				if alt := mset.insert(fld); alt != nil {
   694  					// Struct fields should already be unique, so we should only
   695  					// encounter an alternate via collision with a method name.
   696  					_ = alt.(*Func)
   697  
   698  					// For historical consistency, we report the primary error on the
   699  					// method, and the alt decl on the field.
   700  					var err error_
   701  					err.errorf(alt, "field and method with the same name %s", fld.name)
   702  					err.recordAltDecl(fld)
   703  					check.report(&err)
   704  				}
   705  			}
   706  		}
   707  	}
   708  }
   709  
   710  func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
   711  	assert(obj.typ == nil)
   712  
   713  	// func declarations cannot use iota
   714  	assert(check.iota == nil)
   715  
   716  	sig := new(Signature)
   717  	obj.typ = sig // guard against cycles
   718  
   719  	// Avoid cycle error when referring to method while type-checking the signature.
   720  	// This avoids a nuisance in the best case (non-parameterized receiver type) and
   721  	// since the method is not a type, we get an error. If we have a parameterized
   722  	// receiver type, instantiating the receiver type leads to the instantiation of
   723  	// its methods, and we don't want a cycle error in that case.
   724  	// TODO(gri) review if this is correct and/or whether we still need this?
   725  	saved := obj.color_
   726  	obj.color_ = black
   727  	fdecl := decl.fdecl
   728  	check.funcType(sig, fdecl.Recv, fdecl.TParamList, fdecl.Type)
   729  	obj.color_ = saved
   730  
   731  	if len(fdecl.TParamList) > 0 && fdecl.Body == nil {
   732  		check.softErrorf(fdecl, "parameterized function is missing function body")
   733  	}
   734  
   735  	// function body must be type-checked after global declarations
   736  	// (functions implemented elsewhere have no body)
   737  	if !check.conf.IgnoreFuncBodies && fdecl.Body != nil {
   738  		check.later(func() {
   739  			check.funcBody(decl, obj.name, sig, fdecl.Body, nil)
   740  		})
   741  	}
   742  }
   743  
   744  func (check *Checker) declStmt(list []syntax.Decl) {
   745  	pkg := check.pkg
   746  
   747  	first := -1                // index of first ConstDecl in the current group, or -1
   748  	var last *syntax.ConstDecl // last ConstDecl with init expressions, or nil
   749  	for index, decl := range list {
   750  		if _, ok := decl.(*syntax.ConstDecl); !ok {
   751  			first = -1 // we're not in a constant declaration
   752  		}
   753  
   754  		switch s := decl.(type) {
   755  		case *syntax.ConstDecl:
   756  			top := len(check.delayed)
   757  
   758  			// iota is the index of the current constDecl within the group
   759  			if first < 0 || s.Group == nil || list[index-1].(*syntax.ConstDecl).Group != s.Group {
   760  				first = index
   761  				last = nil
   762  			}
   763  			iota := constant.MakeInt64(int64(index - first))
   764  
   765  			// determine which initialization expressions to use
   766  			inherited := true
   767  			switch {
   768  			case s.Type != nil || s.Values != nil:
   769  				last = s
   770  				inherited = false
   771  			case last == nil:
   772  				last = new(syntax.ConstDecl) // make sure last exists
   773  				inherited = false
   774  			}
   775  
   776  			// declare all constants
   777  			lhs := make([]*Const, len(s.NameList))
   778  			values := unpackExpr(last.Values)
   779  			for i, name := range s.NameList {
   780  				obj := NewConst(name.Pos(), pkg, name.Value, nil, iota)
   781  				lhs[i] = obj
   782  
   783  				var init syntax.Expr
   784  				if i < len(values) {
   785  					init = values[i]
   786  				}
   787  
   788  				check.constDecl(obj, last.Type, init, inherited)
   789  			}
   790  
   791  			// Constants must always have init values.
   792  			check.arity(s.Pos(), s.NameList, values, true, inherited)
   793  
   794  			// process function literals in init expressions before scope changes
   795  			check.processDelayed(top)
   796  
   797  			// spec: "The scope of a constant or variable identifier declared
   798  			// inside a function begins at the end of the ConstSpec or VarSpec
   799  			// (ShortVarDecl for short variable declarations) and ends at the
   800  			// end of the innermost containing block."
   801  			scopePos := syntax.EndPos(s)
   802  			for i, name := range s.NameList {
   803  				check.declare(check.scope, name, lhs[i], scopePos)
   804  			}
   805  
   806  		case *syntax.VarDecl:
   807  			top := len(check.delayed)
   808  
   809  			lhs0 := make([]*Var, len(s.NameList))
   810  			for i, name := range s.NameList {
   811  				lhs0[i] = NewVar(name.Pos(), pkg, name.Value, nil)
   812  			}
   813  
   814  			// initialize all variables
   815  			values := unpackExpr(s.Values)
   816  			for i, obj := range lhs0 {
   817  				var lhs []*Var
   818  				var init syntax.Expr
   819  				switch len(values) {
   820  				case len(s.NameList):
   821  					// lhs and rhs match
   822  					init = values[i]
   823  				case 1:
   824  					// rhs is expected to be a multi-valued expression
   825  					lhs = lhs0
   826  					init = values[0]
   827  				default:
   828  					if i < len(values) {
   829  						init = values[i]
   830  					}
   831  				}
   832  				check.varDecl(obj, lhs, s.Type, init)
   833  				if len(values) == 1 {
   834  					// If we have a single lhs variable we are done either way.
   835  					// If we have a single rhs expression, it must be a multi-
   836  					// valued expression, in which case handling the first lhs
   837  					// variable will cause all lhs variables to have a type
   838  					// assigned, and we are done as well.
   839  					if debug {
   840  						for _, obj := range lhs0 {
   841  							assert(obj.typ != nil)
   842  						}
   843  					}
   844  					break
   845  				}
   846  			}
   847  
   848  			// If we have no type, we must have values.
   849  			if s.Type == nil || values != nil {
   850  				check.arity(s.Pos(), s.NameList, values, false, false)
   851  			}
   852  
   853  			// process function literals in init expressions before scope changes
   854  			check.processDelayed(top)
   855  
   856  			// declare all variables
   857  			// (only at this point are the variable scopes (parents) set)
   858  			scopePos := syntax.EndPos(s) // see constant declarations
   859  			for i, name := range s.NameList {
   860  				// see constant declarations
   861  				check.declare(check.scope, name, lhs0[i], scopePos)
   862  			}
   863  
   864  		case *syntax.TypeDecl:
   865  			obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Value, nil)
   866  			// spec: "The scope of a type identifier declared inside a function
   867  			// begins at the identifier in the TypeSpec and ends at the end of
   868  			// the innermost containing block."
   869  			scopePos := s.Name.Pos()
   870  			check.declare(check.scope, s.Name, obj, scopePos)
   871  			// mark and unmark type before calling typeDecl; its type is still nil (see Checker.objDecl)
   872  			obj.setColor(grey + color(check.push(obj)))
   873  			check.typeDecl(obj, s, nil)
   874  			check.pop().setColor(black)
   875  
   876  		default:
   877  			check.errorf(s, invalidAST+"unknown syntax.Decl node %T", s)
   878  		}
   879  	}
   880  }
   881
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