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  	"slices"
    13  )
    14  
    15  func (check *Checker) declare(scope *Scope, id *syntax.Name, obj Object, pos syntax.Pos) {
    16  	// spec: "The blank identifier, represented by the underscore
    17  	// character _, may be used in a declaration like any other
    18  	// identifier but the declaration does not introduce a new
    19  	// binding."
    20  	if obj.Name() != "_" {
    21  		if alt := scope.Insert(obj); alt != nil {
    22  			err := check.newError(DuplicateDecl)
    23  			err.addf(obj, "%s redeclared in this block", obj.Name())
    24  			err.addAltDecl(alt)
    25  			err.report()
    26  			return
    27  		}
    28  		obj.setScopePos(pos)
    29  	}
    30  	if id != nil {
    31  		check.recordDef(id, obj)
    32  	}
    33  }
    34  
    35  // pathString returns a string of the form a->b-> ... ->g for a path [a, b, ... g].
    36  func pathString(path []Object) string {
    37  	var s string
    38  	for i, p := range path {
    39  		if i > 0 {
    40  			s += "->"
    41  		}
    42  		s += p.Name()
    43  	}
    44  	return s
    45  }
    46  
    47  // objDecl type-checks the declaration of obj in its respective (file) environment.
    48  func (check *Checker) objDecl(obj Object) {
    49  	if tracePos {
    50  		check.pushPos(obj.Pos())
    51  		defer func() {
    52  			// If we're panicking, keep stack of source positions.
    53  			if p := recover(); p != nil {
    54  				panic(p)
    55  			}
    56  			check.popPos()
    57  		}()
    58  	}
    59  
    60  	if check.conf.Trace && obj.Type() == nil {
    61  		if check.indent == 0 {
    62  			fmt.Println() // empty line between top-level objects for readability
    63  		}
    64  		check.trace(obj.Pos(), "-- checking %s (objPath = %s)", obj, pathString(check.objPath))
    65  		check.indent++
    66  		defer func() {
    67  			check.indent--
    68  			check.trace(obj.Pos(), "=> %s", obj)
    69  		}()
    70  	}
    71  
    72  	// Checking the declaration of an object means determining its type
    73  	// (and also its value for constants). An object (and thus its type)
    74  	// may be in 1 of 3 states:
    75  	//
    76  	// - not in Checker.objPathIdx and type == nil : type is not yet known (white)
    77  	// -     in Checker.objPathIdx                 : type is pending       (grey)
    78  	// - not in Checker.objPathIdx and type != nil : type is known         (black)
    79  	//
    80  	// During type-checking, an object changes from white to grey to black.
    81  	// Predeclared objects start as black (their type is known without checking).
    82  	//
    83  	// A black object may only depend on (refer to) to other black objects. White
    84  	// and grey objects may depend on white or black objects. A dependency on a
    85  	// grey object indicates a (possibly invalid) cycle.
    86  	//
    87  	// When an object is marked grey, it is pushed onto the object path (a stack)
    88  	// and its index in the path is recorded in the path index map. It is popped
    89  	// and removed from the map when its type is determined (and marked black).
    90  
    91  	// If this object is grey, we have a (possibly invalid) cycle. This is signaled
    92  	// by a non-nil type for the object, except for constants and variables whose
    93  	// type may be non-nil (known), or nil if it depends on a not-yet known
    94  	// initialization value.
    95  	//
    96  	// In the former case, set the type to Typ[Invalid] because we have an
    97  	// initialization cycle. The cycle error will be reported later, when
    98  	// determining initialization order.
    99  	//
   100  	// TODO(gri) Report cycle here and simplify initialization order code.
   101  	if _, ok := check.objPathIdx[obj]; ok {
   102  		switch obj := obj.(type) {
   103  		case *Const, *Var:
   104  			if !check.validCycle(obj) || obj.Type() == nil {
   105  				obj.setType(Typ[Invalid])
   106  			}
   107  		case *TypeName:
   108  			if !check.validCycle(obj) {
   109  				obj.setType(Typ[Invalid])
   110  			}
   111  		case *Func:
   112  			if !check.validCycle(obj) {
   113  				// Don't set type to Typ[Invalid]; plenty of code asserts that
   114  				// functions have a *Signature type. Instead, leave the type
   115  				// as an empty signature, which makes it impossible to
   116  				// initialize a variable with the function.
   117  			}
   118  		default:
   119  			panic("unreachable")
   120  		}
   121  
   122  		assert(obj.Type() != nil)
   123  		return
   124  	}
   125  
   126  	if obj.Type() != nil { // black, meaning it's already type-checked
   127  		return
   128  	}
   129  
   130  	// white, meaning it must be type-checked
   131  
   132  	check.push(obj)
   133  	defer check.pop()
   134  
   135  	d, ok := check.objMap[obj]
   136  	assert(ok)
   137  
   138  	// save/restore current environment and set up object environment
   139  	defer func(env environment) {
   140  		check.environment = env
   141  	}(check.environment)
   142  	check.environment = environment{scope: d.file, version: d.version}
   143  
   144  	// Const and var declarations must not have initialization
   145  	// cycles. We track them by remembering the current declaration
   146  	// in check.decl. Initialization expressions depending on other
   147  	// consts, vars, or functions, add dependencies to the current
   148  	// check.decl.
   149  	switch obj := obj.(type) {
   150  	case *Const:
   151  		check.decl = d // new package-level const decl
   152  		check.constDecl(obj, d.vtyp, d.init, d.inherited)
   153  	case *Var:
   154  		check.decl = d // new package-level var decl
   155  		check.varDecl(obj, d.lhs, d.vtyp, d.init)
   156  	case *TypeName:
   157  		// invalid recursive types are detected via path
   158  		check.typeDecl(obj, d.tdecl)
   159  		check.collectMethods(obj) // methods can only be added to top-level types
   160  	case *Func:
   161  		// functions may be recursive - no need to track dependencies
   162  		check.funcDecl(obj, d)
   163  	default:
   164  		panic("unreachable")
   165  	}
   166  }
   167  
   168  // validCycle reports whether the cycle starting with obj is valid and
   169  // reports an error if it is not.
   170  func (check *Checker) validCycle(obj Object) (valid bool) {
   171  	// The object map contains the package scope objects and the non-interface methods.
   172  	if debug {
   173  		info := check.objMap[obj]
   174  		inObjMap := info != nil && (info.fdecl == nil || info.fdecl.Recv == nil) // exclude methods
   175  		isPkgObj := obj.Parent() == check.pkg.scope
   176  		if isPkgObj != inObjMap {
   177  			check.dump("%v: inconsistent object map for %s (isPkgObj = %v, inObjMap = %v)", obj.Pos(), obj, isPkgObj, inObjMap)
   178  			panic("unreachable")
   179  		}
   180  	}
   181  
   182  	// Count cycle objects.
   183  	start, found := check.objPathIdx[obj]
   184  	assert(found)
   185  	cycle := check.objPath[start:]
   186  	tparCycle := false // if set, the cycle is through a type parameter list
   187  	nval := 0          // number of (constant or variable) values in the cycle
   188  	ndef := 0          // number of type definitions in the cycle
   189  loop:
   190  	for _, obj := range cycle {
   191  		switch obj := obj.(type) {
   192  		case *Const, *Var:
   193  			nval++
   194  		case *TypeName:
   195  			// If we reach a generic type that is part of a cycle
   196  			// and we are in a type parameter list, we have a cycle
   197  			// through a type parameter list.
   198  			if check.inTParamList && isGeneric(obj.typ) {
   199  				tparCycle = true
   200  				break loop
   201  			}
   202  			if !obj.IsAlias() {
   203  				ndef++
   204  			}
   205  		case *Func:
   206  			// ignored for now
   207  		default:
   208  			panic("unreachable")
   209  		}
   210  	}
   211  
   212  	if check.conf.Trace {
   213  		check.trace(obj.Pos(), "## cycle detected: objPath = %s->%s (len = %d)", pathString(cycle), obj.Name(), len(cycle))
   214  		if tparCycle {
   215  			check.trace(obj.Pos(), "## cycle contains: generic type in a type parameter list")
   216  		} else {
   217  			check.trace(obj.Pos(), "## cycle contains: %d values, %d type definitions", nval, ndef)
   218  		}
   219  		defer func() {
   220  			if valid {
   221  				check.trace(obj.Pos(), "=> cycle is valid")
   222  			} else {
   223  				check.trace(obj.Pos(), "=> error: cycle is invalid")
   224  			}
   225  		}()
   226  	}
   227  
   228  	// Cycles through type parameter lists are ok (go.dev/issue/68162).
   229  	if tparCycle {
   230  		return true
   231  	}
   232  
   233  	// A cycle involving only constants and variables is invalid but we
   234  	// ignore them here because they are reported via the initialization
   235  	// cycle check.
   236  	if nval == len(cycle) {
   237  		return true
   238  	}
   239  
   240  	// A cycle involving only types (and possibly functions) must have at least
   241  	// one type definition to be permitted: If there is no type definition, we
   242  	// have a sequence of alias type names which will expand ad infinitum.
   243  	if nval == 0 && ndef > 0 {
   244  		return true
   245  	}
   246  
   247  	check.cycleError(cycle, firstInSrc(cycle))
   248  	return false
   249  }
   250  
   251  // cycleError reports a declaration cycle starting with the object at cycle[start].
   252  func (check *Checker) cycleError(cycle []Object, start int) {
   253  	// name returns the (possibly qualified) object name.
   254  	// This is needed because with generic types, cycles
   255  	// may refer to imported types. See go.dev/issue/50788.
   256  	// TODO(gri) This functionality is used elsewhere. Factor it out.
   257  	name := func(obj Object) string {
   258  		// include any type arguments in the reported error message
   259  		if n := asNamed(obj.Type()); n != nil && n.inst != nil {
   260  			return TypeString(n, check.qualifier)
   261  		}
   262  		return packagePrefix(obj.Pkg(), check.qualifier) + obj.Name()
   263  	}
   264  
   265  	// If obj is a type alias, mark it as valid (not broken) in order to avoid follow-on errors.
   266  	obj := cycle[start]
   267  	tname, _ := obj.(*TypeName)
   268  	if tname != nil {
   269  		if a, ok := tname.Type().(*Alias); ok {
   270  			a.fromRHS = Typ[Invalid]
   271  		}
   272  	}
   273  
   274  	// report a more concise error for self references
   275  	if len(cycle) == 1 {
   276  		if tname != nil {
   277  			check.errorf(obj, InvalidDeclCycle, "invalid recursive type: %s refers to itself", name(obj))
   278  		} else {
   279  			check.errorf(obj, InvalidDeclCycle, "invalid cycle in declaration: %s refers to itself", name(obj))
   280  		}
   281  		return
   282  	}
   283  
   284  	err := check.newError(InvalidDeclCycle)
   285  	if tname != nil {
   286  		err.addf(obj, "invalid recursive type %s", name(obj))
   287  	} else {
   288  		err.addf(obj, "invalid cycle in declaration of %s", name(obj))
   289  	}
   290  	// "cycle[i] refers to cycle[j]" for (i,j) = (s,s+1), (s+1,s+2), ..., (n-1,0), (0,1), ..., (s-1,s) for len(cycle) = n, s = start.
   291  	for i := range cycle {
   292  		next := cycle[(start+i+1)%len(cycle)]
   293  		err.addf(obj, "%s refers to %s", name(obj), name(next))
   294  		obj = next
   295  	}
   296  	err.report()
   297  }
   298  
   299  // firstInSrc reports the index of the object with the "smallest"
   300  // source position in path. path must not be empty.
   301  func firstInSrc(path []Object) int {
   302  	fst, pos := 0, path[0].Pos()
   303  	for i, t := range path[1:] {
   304  		if cmpPos(t.Pos(), pos) < 0 {
   305  			fst, pos = i+1, t.Pos()
   306  		}
   307  	}
   308  	return fst
   309  }
   310  
   311  func (check *Checker) constDecl(obj *Const, typ, init syntax.Expr, inherited bool) {
   312  	assert(obj.typ == nil)
   313  
   314  	// use the correct value of iota and errpos
   315  	defer func(iota constant.Value, errpos syntax.Pos) {
   316  		check.iota = iota
   317  		check.errpos = errpos
   318  	}(check.iota, check.errpos)
   319  	check.iota = obj.val
   320  	check.errpos = nopos
   321  
   322  	// provide valid constant value under all circumstances
   323  	obj.val = constant.MakeUnknown()
   324  
   325  	// determine type, if any
   326  	if typ != nil {
   327  		t := check.typ(typ)
   328  		if !isConstType(t) {
   329  			// don't report an error if the type is an invalid C (defined) type
   330  			// (go.dev/issue/22090)
   331  			if isValid(t.Underlying()) {
   332  				check.errorf(typ, InvalidConstType, "invalid constant type %s", t)
   333  			}
   334  			obj.typ = Typ[Invalid]
   335  			return
   336  		}
   337  		obj.typ = t
   338  	}
   339  
   340  	// check initialization
   341  	var x operand
   342  	if init != nil {
   343  		if inherited {
   344  			// The initialization expression is inherited from a previous
   345  			// constant declaration, and (error) positions refer to that
   346  			// expression and not the current constant declaration. Use
   347  			// the constant identifier position for any errors during
   348  			// init expression evaluation since that is all we have
   349  			// (see issues go.dev/issue/42991, go.dev/issue/42992).
   350  			check.errpos = obj.pos
   351  		}
   352  		check.expr(nil, &x, init)
   353  	}
   354  	check.initConst(obj, &x)
   355  }
   356  
   357  func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.Expr) {
   358  	assert(obj.typ == nil)
   359  
   360  	// determine type, if any
   361  	if typ != nil {
   362  		obj.typ = check.varType(typ)
   363  		// We cannot spread the type to all lhs variables if there
   364  		// are more than one since that would mark them as checked
   365  		// (see Checker.objDecl) and the assignment of init exprs,
   366  		// if any, would not be checked.
   367  		//
   368  		// TODO(gri) If we have no init expr, we should distribute
   369  		// a given type otherwise we need to re-evaluate the type
   370  		// expr for each lhs variable, leading to duplicate work.
   371  	}
   372  
   373  	// check initialization
   374  	if init == nil {
   375  		if typ == nil {
   376  			// error reported before by arityMatch
   377  			obj.typ = Typ[Invalid]
   378  		}
   379  		return
   380  	}
   381  
   382  	if lhs == nil || len(lhs) == 1 {
   383  		assert(lhs == nil || lhs[0] == obj)
   384  		var x operand
   385  		check.expr(newTarget(obj.typ, obj.name), &x, init)
   386  		check.initVar(obj, &x, "variable declaration")
   387  		return
   388  	}
   389  
   390  	if debug {
   391  		// obj must be one of lhs
   392  		if !slices.Contains(lhs, obj) {
   393  			panic("inconsistent lhs")
   394  		}
   395  	}
   396  
   397  	// We have multiple variables on the lhs and one init expr.
   398  	// Make sure all variables have been given the same type if
   399  	// one was specified, otherwise they assume the type of the
   400  	// init expression values (was go.dev/issue/15755).
   401  	if typ != nil {
   402  		for _, lhs := range lhs {
   403  			lhs.typ = obj.typ
   404  		}
   405  	}
   406  
   407  	check.initVars(lhs, []syntax.Expr{init}, nil)
   408  }
   409  
   410  // isImportedConstraint reports whether typ is an imported type constraint.
   411  func (check *Checker) isImportedConstraint(typ Type) bool {
   412  	named := asNamed(typ)
   413  	if named == nil || named.obj.pkg == check.pkg || named.obj.pkg == nil {
   414  		return false
   415  	}
   416  	u, _ := named.Underlying().(*Interface)
   417  	return u != nil && !u.IsMethodSet()
   418  }
   419  
   420  func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl) {
   421  	assert(obj.typ == nil)
   422  
   423  	// Only report a version error if we have not reported one already.
   424  	versionErr := false
   425  
   426  	var rhs Type
   427  	check.later(func() {
   428  		if t := asNamed(obj.typ); t != nil { // type may be invalid
   429  			check.validType(t)
   430  		}
   431  		// If typ is local, an error was already reported where typ is specified/defined.
   432  		_ = !versionErr && check.isImportedConstraint(rhs) && check.verifyVersionf(tdecl.Type, go1_18, "using type constraint %s", rhs)
   433  	}).describef(obj, "validType(%s)", obj.Name())
   434  
   435  	// First type parameter, or nil.
   436  	var tparam0 *syntax.Field
   437  	if len(tdecl.TParamList) > 0 {
   438  		tparam0 = tdecl.TParamList[0]
   439  	}
   440  
   441  	// alias declaration
   442  	if tdecl.Alias {
   443  		// Report highest version requirement first so that fixing a version issue
   444  		// avoids possibly two -lang changes (first to Go 1.9 and then to Go 1.23).
   445  		if !versionErr && tparam0 != nil && !check.verifyVersionf(tparam0, go1_23, "generic type alias") {
   446  			versionErr = true
   447  		}
   448  		if !versionErr && !check.verifyVersionf(tdecl, go1_9, "type alias") {
   449  			versionErr = true
   450  		}
   451  
   452  		alias := check.newAlias(obj, nil)
   453  
   454  		// If we could not type the RHS, set it to invalid. This should
   455  		// only ever happen if we panic before setting.
   456  		defer func() {
   457  			if alias.fromRHS == nil {
   458  				alias.fromRHS = Typ[Invalid]
   459  				unalias(alias)
   460  			}
   461  		}()
   462  
   463  		// handle type parameters even if not allowed (Alias type is supported)
   464  		if tparam0 != nil {
   465  			check.openScope(tdecl, "type parameters")
   466  			defer check.closeScope()
   467  			check.collectTypeParams(&alias.tparams, tdecl.TParamList)
   468  		}
   469  
   470  		rhs = check.declaredType(tdecl.Type, obj)
   471  		assert(rhs != nil)
   472  		alias.fromRHS = rhs
   473  
   474  		// spec: In an alias declaration the given type cannot be a type parameter declared in the same declaration."
   475  		// (see also go.dev/issue/75884, go.dev/issue/#75885)
   476  		if tpar, ok := rhs.(*TypeParam); ok && alias.tparams != nil && slices.Index(alias.tparams.list(), tpar) >= 0 {
   477  			check.error(tdecl.Type, MisplacedTypeParam, "cannot use type parameter declared in alias declaration as RHS")
   478  			alias.fromRHS = Typ[Invalid]
   479  		}
   480  
   481  		return
   482  	}
   483  
   484  	// type definition or generic type declaration
   485  	if !versionErr && tparam0 != nil && !check.verifyVersionf(tparam0, go1_18, "type parameter") {
   486  		versionErr = true
   487  	}
   488  
   489  	named := check.newNamed(obj, nil, nil)
   490  	if tdecl.TParamList != nil {
   491  		check.openScope(tdecl, "type parameters")
   492  		defer check.closeScope()
   493  		check.collectTypeParams(&named.tparams, tdecl.TParamList)
   494  	}
   495  
   496  	rhs = check.declaredType(tdecl.Type, obj)
   497  	assert(rhs != nil)
   498  	named.fromRHS = rhs
   499  
   500  	// spec: "In a type definition the given type cannot be a type parameter."
   501  	// (See also go.dev/issue/45639.)
   502  	if isTypeParam(rhs) {
   503  		check.error(tdecl.Type, MisplacedTypeParam, "cannot use a type parameter as RHS in type declaration")
   504  		named.fromRHS = Typ[Invalid]
   505  	}
   506  }
   507  
   508  func (check *Checker) collectTypeParams(dst **TypeParamList, list []*syntax.Field) {
   509  	tparams := make([]*TypeParam, len(list))
   510  
   511  	// Declare type parameters up-front.
   512  	// The scope of type parameters starts at the beginning of the type parameter
   513  	// list (so we can have mutually recursive parameterized type bounds).
   514  	if len(list) > 0 {
   515  		scopePos := list[0].Pos()
   516  		for i, f := range list {
   517  			tparams[i] = check.declareTypeParam(f.Name, scopePos)
   518  		}
   519  	}
   520  
   521  	// Set the type parameters before collecting the type constraints because
   522  	// the parameterized type may be used by the constraints (go.dev/issue/47887).
   523  	// Example: type T[P T[P]] interface{}
   524  	*dst = bindTParams(tparams)
   525  
   526  	// Signal to cycle detection that we are in a type parameter list.
   527  	// We can only be inside one type parameter list at any given time:
   528  	// function closures may appear inside a type parameter list but they
   529  	// cannot be generic, and their bodies are processed in delayed and
   530  	// sequential fashion. Note that with each new declaration, we save
   531  	// the existing environment and restore it when done; thus inTParamList
   532  	// is true exactly only when we are in a specific type parameter list.
   533  	assert(!check.inTParamList)
   534  	check.inTParamList = true
   535  	defer func() {
   536  		check.inTParamList = false
   537  	}()
   538  
   539  	// Keep track of bounds for later validation.
   540  	var bound Type
   541  	for i, f := range list {
   542  		// Optimization: Re-use the previous type bound if it hasn't changed.
   543  		// This also preserves the grouped output of type parameter lists
   544  		// when printing type strings.
   545  		if i == 0 || f.Type != list[i-1].Type {
   546  			bound = check.bound(f.Type)
   547  			if isTypeParam(bound) {
   548  				// We may be able to allow this since it is now well-defined what
   549  				// the underlying type and thus type set of a type parameter is.
   550  				// But we may need some additional form of cycle detection within
   551  				// type parameter lists.
   552  				check.error(f.Type, MisplacedTypeParam, "cannot use a type parameter as constraint")
   553  				bound = Typ[Invalid]
   554  			}
   555  		}
   556  		tparams[i].bound = bound
   557  	}
   558  }
   559  
   560  func (check *Checker) bound(x syntax.Expr) Type {
   561  	// A type set literal of the form ~T and A|B may only appear as constraint;
   562  	// embed it in an implicit interface so that only interface type-checking
   563  	// needs to take care of such type expressions.
   564  	if op, _ := x.(*syntax.Operation); op != nil && (op.Op == syntax.Tilde || op.Op == syntax.Or) {
   565  		t := check.typ(&syntax.InterfaceType{MethodList: []*syntax.Field{{Type: x}}})
   566  		// mark t as implicit interface if all went well
   567  		if t, _ := t.(*Interface); t != nil {
   568  			t.implicit = true
   569  		}
   570  		return t
   571  	}
   572  	return check.typ(x)
   573  }
   574  
   575  func (check *Checker) declareTypeParam(name *syntax.Name, scopePos syntax.Pos) *TypeParam {
   576  	// Use Typ[Invalid] for the type constraint to ensure that a type
   577  	// is present even if the actual constraint has not been assigned
   578  	// yet.
   579  	// TODO(gri) Need to systematically review all uses of type parameter
   580  	//           constraints to make sure we don't rely on them if they
   581  	//           are not properly set yet.
   582  	tname := NewTypeName(name.Pos(), check.pkg, name.Value, nil)
   583  	tpar := check.newTypeParam(tname, Typ[Invalid]) // assigns type to tname as a side-effect
   584  	check.declare(check.scope, name, tname, scopePos)
   585  	return tpar
   586  }
   587  
   588  func (check *Checker) collectMethods(obj *TypeName) {
   589  	// get associated methods
   590  	// (Checker.collectObjects only collects methods with non-blank names;
   591  	// Checker.resolveBaseTypeName ensures that obj is not an alias name
   592  	// if it has attached methods.)
   593  	methods := check.methods[obj]
   594  	if methods == nil {
   595  		return
   596  	}
   597  	delete(check.methods, obj)
   598  	assert(!check.objMap[obj].tdecl.Alias) // don't use TypeName.IsAlias (requires fully set up object)
   599  
   600  	// use an objset to check for name conflicts
   601  	var mset objset
   602  
   603  	// spec: "If the base type is a struct type, the non-blank method
   604  	// and field names must be distinct."
   605  	base := asNamed(obj.typ) // shouldn't fail but be conservative
   606  	if base != nil {
   607  		assert(base.TypeArgs().Len() == 0) // collectMethods should not be called on an instantiated type
   608  
   609  		// See go.dev/issue/52529: we must delay the expansion of underlying here, as
   610  		// base may not be fully set-up.
   611  		check.later(func() {
   612  			check.checkFieldUniqueness(base)
   613  		}).describef(obj, "verifying field uniqueness for %v", base)
   614  
   615  		// Checker.Files may be called multiple times; additional package files
   616  		// may add methods to already type-checked types. Add pre-existing methods
   617  		// so that we can detect redeclarations.
   618  		for i := 0; i < base.NumMethods(); i++ {
   619  			m := base.Method(i)
   620  			assert(m.name != "_")
   621  			assert(mset.insert(m) == nil)
   622  		}
   623  	}
   624  
   625  	// add valid methods
   626  	for _, m := range methods {
   627  		// spec: "For a base type, the non-blank names of methods bound
   628  		// to it must be unique."
   629  		assert(m.name != "_")
   630  		if alt := mset.insert(m); alt != nil {
   631  			if alt.Pos().IsKnown() {
   632  				check.errorf(m.pos, DuplicateMethod, "method %s.%s already declared at %v", obj.Name(), m.name, alt.Pos())
   633  			} else {
   634  				check.errorf(m.pos, DuplicateMethod, "method %s.%s already declared", obj.Name(), m.name)
   635  			}
   636  			continue
   637  		}
   638  
   639  		if base != nil {
   640  			base.AddMethod(m)
   641  		}
   642  	}
   643  }
   644  
   645  func (check *Checker) checkFieldUniqueness(base *Named) {
   646  	if t, _ := base.Underlying().(*Struct); t != nil {
   647  		var mset objset
   648  		for i := 0; i < base.NumMethods(); i++ {
   649  			m := base.Method(i)
   650  			assert(m.name != "_")
   651  			assert(mset.insert(m) == nil)
   652  		}
   653  
   654  		// Check that any non-blank field names of base are distinct from its
   655  		// method names.
   656  		for _, fld := range t.fields {
   657  			if fld.name != "_" {
   658  				if alt := mset.insert(fld); alt != nil {
   659  					// Struct fields should already be unique, so we should only
   660  					// encounter an alternate via collision with a method name.
   661  					_ = alt.(*Func)
   662  
   663  					// For historical consistency, we report the primary error on the
   664  					// method, and the alt decl on the field.
   665  					err := check.newError(DuplicateFieldAndMethod)
   666  					err.addf(alt, "field and method with the same name %s", fld.name)
   667  					err.addAltDecl(fld)
   668  					err.report()
   669  				}
   670  			}
   671  		}
   672  	}
   673  }
   674  
   675  func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
   676  	assert(obj.typ == nil)
   677  
   678  	// func declarations cannot use iota
   679  	assert(check.iota == nil)
   680  
   681  	sig := new(Signature)
   682  	obj.typ = sig // guard against cycles
   683  
   684  	fdecl := decl.fdecl
   685  	check.funcType(sig, fdecl.Recv, fdecl.TParamList, fdecl.Type)
   686  
   687  	if fdecl.Pragma != nil {
   688  		if p, ok := fdecl.Pragma.(interface{ Nointerface() bool }); ok && p.Nointerface() {
   689  			obj.nointerface = true
   690  		}
   691  	}
   692  
   693  	// Set the scope's extent to the complete "func (...) { ... }"
   694  	// so that Scope.Innermost works correctly.
   695  	sig.scope.pos = fdecl.Pos()
   696  	sig.scope.end = syntax.EndPos(fdecl)
   697  
   698  	if len(fdecl.TParamList) > 0 && fdecl.Body == nil {
   699  		check.softErrorf(fdecl, BadDecl, "generic function is missing function body")
   700  	}
   701  
   702  	// function body must be type-checked after global declarations
   703  	// (functions implemented elsewhere have no body)
   704  	if !check.conf.IgnoreFuncBodies && fdecl.Body != nil {
   705  		check.later(func() {
   706  			check.funcBody(decl, obj.name, sig, fdecl.Body, nil)
   707  		}).describef(obj, "func %s", obj.name)
   708  	}
   709  }
   710  
   711  func (check *Checker) declStmt(list []syntax.Decl) {
   712  	pkg := check.pkg
   713  
   714  	first := -1                // index of first ConstDecl in the current group, or -1
   715  	var last *syntax.ConstDecl // last ConstDecl with init expressions, or nil
   716  	for index, decl := range list {
   717  		if _, ok := decl.(*syntax.ConstDecl); !ok {
   718  			first = -1 // we're not in a constant declaration
   719  		}
   720  
   721  		switch s := decl.(type) {
   722  		case *syntax.ConstDecl:
   723  			top := len(check.delayed)
   724  
   725  			// iota is the index of the current constDecl within the group
   726  			if first < 0 || s.Group == nil || list[index-1].(*syntax.ConstDecl).Group != s.Group {
   727  				first = index
   728  				last = nil
   729  			}
   730  			iota := constant.MakeInt64(int64(index - first))
   731  
   732  			// determine which initialization expressions to use
   733  			inherited := true
   734  			switch {
   735  			case s.Type != nil || s.Values != nil:
   736  				last = s
   737  				inherited = false
   738  			case last == nil:
   739  				last = new(syntax.ConstDecl) // make sure last exists
   740  				inherited = false
   741  			}
   742  
   743  			// declare all constants
   744  			lhs := make([]*Const, len(s.NameList))
   745  			values := syntax.UnpackListExpr(last.Values)
   746  			for i, name := range s.NameList {
   747  				obj := NewConst(name.Pos(), pkg, name.Value, nil, iota)
   748  				lhs[i] = obj
   749  
   750  				var init syntax.Expr
   751  				if i < len(values) {
   752  					init = values[i]
   753  				}
   754  
   755  				check.constDecl(obj, last.Type, init, inherited)
   756  			}
   757  
   758  			// Constants must always have init values.
   759  			check.arity(s.Pos(), s.NameList, values, true, inherited)
   760  
   761  			// process function literals in init expressions before scope changes
   762  			check.processDelayed(top)
   763  
   764  			// spec: "The scope of a constant or variable identifier declared
   765  			// inside a function begins at the end of the ConstSpec or VarSpec
   766  			// (ShortVarDecl for short variable declarations) and ends at the
   767  			// end of the innermost containing block."
   768  			scopePos := syntax.EndPos(s)
   769  			for i, name := range s.NameList {
   770  				check.declare(check.scope, name, lhs[i], scopePos)
   771  			}
   772  
   773  		case *syntax.VarDecl:
   774  			top := len(check.delayed)
   775  
   776  			lhs0 := make([]*Var, len(s.NameList))
   777  			for i, name := range s.NameList {
   778  				lhs0[i] = newVar(LocalVar, name.Pos(), pkg, name.Value, nil)
   779  			}
   780  
   781  			// initialize all variables
   782  			values := syntax.UnpackListExpr(s.Values)
   783  			for i, obj := range lhs0 {
   784  				var lhs []*Var
   785  				var init syntax.Expr
   786  				switch len(values) {
   787  				case len(s.NameList):
   788  					// lhs and rhs match
   789  					init = values[i]
   790  				case 1:
   791  					// rhs is expected to be a multi-valued expression
   792  					lhs = lhs0
   793  					init = values[0]
   794  				default:
   795  					if i < len(values) {
   796  						init = values[i]
   797  					}
   798  				}
   799  				check.varDecl(obj, lhs, s.Type, init)
   800  				if len(values) == 1 {
   801  					// If we have a single lhs variable we are done either way.
   802  					// If we have a single rhs expression, it must be a multi-
   803  					// valued expression, in which case handling the first lhs
   804  					// variable will cause all lhs variables to have a type
   805  					// assigned, and we are done as well.
   806  					if debug {
   807  						for _, obj := range lhs0 {
   808  							assert(obj.typ != nil)
   809  						}
   810  					}
   811  					break
   812  				}
   813  			}
   814  
   815  			// If we have no type, we must have values.
   816  			if s.Type == nil || values != nil {
   817  				check.arity(s.Pos(), s.NameList, values, false, false)
   818  			}
   819  
   820  			// process function literals in init expressions before scope changes
   821  			check.processDelayed(top)
   822  
   823  			// declare all variables
   824  			// (only at this point are the variable scopes (parents) set)
   825  			scopePos := syntax.EndPos(s) // see constant declarations
   826  			for i, name := range s.NameList {
   827  				// see constant declarations
   828  				check.declare(check.scope, name, lhs0[i], scopePos)
   829  			}
   830  
   831  		case *syntax.TypeDecl:
   832  			obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Value, nil)
   833  			// spec: "The scope of a type identifier declared inside a function
   834  			// begins at the identifier in the TypeSpec and ends at the end of
   835  			// the innermost containing block."
   836  			scopePos := s.Name.Pos()
   837  			check.declare(check.scope, s.Name, obj, scopePos)
   838  			check.push(obj) // mark as grey
   839  			check.typeDecl(obj, s)
   840  			check.pop()
   841  
   842  		default:
   843  			check.errorf(s, InvalidSyntaxTree, "unknown syntax.Decl node %T", s)
   844  		}
   845  	}
   846  }
   847
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