src/go/types/typexpr.go - go - Git at Google

 // Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // This file implements type-checking of identifiers and type expressions.

 package types

 import (
 	"fmt"
 	"go/ast"
 	"go/constant"
 	"go/internal/typeparams"
 	"go/token"
 	"strings"
 )

 // ident type-checks identifier e and initializes x with the value or type of e.
 // If an error occurred, x.mode is set to invalid.
 // For the meaning of def, see Checker.definedType, below.
 // If wantType is set, the identifier e is expected to denote a type.
 //
 func (check *Checker) ident(x *operand, e *ast.Ident, def *Named, wantType bool) {
 	x.mode = invalid
 	x.expr = e

 	// Note that we cannot use check.lookup here because the returned scope
 	// may be different from obj.Parent(). See also Scope.LookupParent doc.
 	scope, obj := check.scope.LookupParent(e.Name, check.pos)
 	switch obj {
 	case nil:
 		if e.Name == "_" {
 			check.error(e, _InvalidBlank, "cannot use _ as value or type")
 		} else {
 			check.errorf(e, _UndeclaredName, "undeclared name: %s", e.Name)
 		}
 		return
 	case universeAny, universeComparable:
 		// complain if necessary
 		if !check.allowVersion(check.pkg, 1, 18) {
 			check.errorf(e, _UndeclaredName, "undeclared name: %s (requires version go1.18 or later)", e.Name)
 			return // avoid follow-on errors
 		}
 		if obj == universeAny {
 			// If we allow "any" for general use, this if-statement can be removed (issue #33232).
 			check.softErrorf(e, _Todo, "cannot use any outside constraint position")
 			// ok to continue
 		}
 	}
 	check.recordUse(e, obj)

 	// Type-check the object.
 	// Only call Checker.objDecl if the object doesn't have a type yet
 	// (in which case we must actually determine it) or the object is a
 	// TypeName and we also want a type (in which case we might detect
 	// a cycle which needs to be reported). Otherwise we can skip the
 	// call and avoid a possible cycle error in favor of the more
 	// informative "not a type/value" error that this function's caller
 	// will issue (see issue #25790).
 	typ := obj.Type()
 	if _, gotType := obj.(*TypeName); typ == nil || gotType && wantType {
 		check.objDecl(obj, def)
 		typ = obj.Type() // type must have been assigned by Checker.objDecl
 	}
 	assert(typ != nil)

 	// The object may have been dot-imported.
 	// If so, mark the respective package as used.
 	// (This code is only needed for dot-imports. Without them,
 	// we only have to mark variables, see *Var case below).
 	if pkgName := check.dotImportMap[dotImportKey{scope, obj.Name()}]; pkgName != nil {
 		pkgName.used = true
 	}

 	switch obj := obj.(type) {
 	case *PkgName:
 		check.errorf(e, _InvalidPkgUse, "use of package %s not in selector", obj.name)
 		return

 	case *Const:
 		check.addDeclDep(obj)
 		if typ == Typ[Invalid] {
 			return
 		}
 		if obj == universeIota {
 			if check.iota == nil {
 				check.errorf(e, _InvalidIota, "cannot use iota outside constant declaration")
 				return
 			}
 			x.val = check.iota
 		} else {
 			x.val = obj.val
 		}
 		assert(x.val != nil)
 		x.mode = constant_

 	case *TypeName:
 		x.mode = typexpr

 	case *Var:
 		// It's ok to mark non-local variables, but ignore variables
 		// from other packages to avoid potential race conditions with
 		// dot-imported variables.
 		if obj.pkg == check.pkg {
 			obj.used = true
 		}
 		check.addDeclDep(obj)
 		if typ == Typ[Invalid] {
 			return
 		}
 		x.mode = variable

 	case *Func:
 		check.addDeclDep(obj)
 		x.mode = value

 	case *Builtin:
 		x.id = obj.id
 		x.mode = builtin

 	case *Nil:
 		x.mode = value

 	default:
 		unreachable()
 	}

 	x.typ = typ
 }

 // typ type-checks the type expression e and returns its type, or Typ[Invalid].
 // The type must not be an (uninstantiated) generic type.
 func (check *Checker) typ(e ast.Expr) Type {
 	return check.definedType(e, nil)
 }

 // varType type-checks the type expression e and returns its type, or Typ[Invalid].
 // The type must not be an (uninstantiated) generic type and it must not be a
 // constraint interface.
 func (check *Checker) varType(e ast.Expr) Type {
 	typ := check.definedType(e, nil)
 	// We don't want to call under() (via asInterface) or complete interfaces while we
 	// are in the middle of type-checking parameter declarations that might belong to
 	// interface methods. Delay this check to the end of type-checking.
 	check.later(func() {
 		if t := asInterface(typ); t != nil {
 			tset := computeInterfaceTypeSet(check, e.Pos(), t) // TODO(gri) is this the correct position?
 			if tset.IsConstraint() {
 				if tset.comparable {
 					check.softErrorf(e, _Todo, "interface is (or embeds) comparable")
 				} else {
 					check.softErrorf(e, _Todo, "interface contains type constraints")
 				}
 			}
 		}
 	})

 	return typ
 }

 // definedType is like typ but also accepts a type name def.
 // If def != nil, e is the type specification for the defined type def, declared
 // in a type declaration, and def.underlying will be set to the type of e before
 // any components of e are type-checked.
 //
 func (check *Checker) definedType(e ast.Expr, def *Named) Type {
 	typ := check.typInternal(e, def)
 	assert(isTyped(typ))
 	if isGeneric(typ) {
 		check.errorf(e, _Todo, "cannot use generic type %s without instantiation", typ)
 		typ = Typ[Invalid]
 	}
 	check.recordTypeAndValue(e, typexpr, typ, nil)
 	return typ
 }

 // genericType is like typ but the type must be an (uninstantiated) generic type.
 func (check *Checker) genericType(e ast.Expr, reportErr bool) Type {
 	typ := check.typInternal(e, nil)
 	assert(isTyped(typ))
 	if typ != Typ[Invalid] && !isGeneric(typ) {
 		if reportErr {
 			check.errorf(e, _Todo, "%s is not a generic type", typ)
 		}
 		typ = Typ[Invalid]
 	}
 	// TODO(gri) what is the correct call below?
 	check.recordTypeAndValue(e, typexpr, typ, nil)
 	return typ
 }

 // goTypeName returns the Go type name for typ and
 // removes any occurrences of "types." from that name.
 func goTypeName(typ Type) string {
 	return strings.ReplaceAll(fmt.Sprintf("%T", typ), "types.", "")
 }

 // typInternal drives type checking of types.
 // Must only be called by definedType or genericType.
 //
 func (check *Checker) typInternal(e0 ast.Expr, def *Named) (T Type) {
 	if trace {
 		check.trace(e0.Pos(), "type %s", e0)
 		check.indent++
 		defer func() {
 			check.indent--
 			var under Type
 			if T != nil {
 				// Calling under() here may lead to endless instantiations.
 				// Test case: type T[P any] *T[P]
 				// TODO(gri) investigate if that's a bug or to be expected
 				// (see also analogous comment in Checker.instantiate).
 				under = safeUnderlying(T)
 			}
 			if T == under {
 				check.trace(e0.Pos(), "=> %s // %s", T, goTypeName(T))
 			} else {
 				check.trace(e0.Pos(), "=> %s (under = %s) // %s", T, under, goTypeName(T))
 			}
 		}()
 	}

 	switch e := e0.(type) {
 	case *ast.BadExpr:
 		// ignore - error reported before

 	case *ast.Ident:
 		var x operand
 		check.ident(&x, e, def, true)

 		switch x.mode {
 		case typexpr:
 			typ := x.typ
 			def.setUnderlying(typ)
 			return typ
 		case invalid:
 			// ignore - error reported before
 		case novalue:
 			check.errorf(&x, _NotAType, "%s used as type", &x)
 		default:
 			check.errorf(&x, _NotAType, "%s is not a type", &x)
 		}

 	case *ast.SelectorExpr:
 		var x operand
 		check.selector(&x, e)

 		switch x.mode {
 		case typexpr:
 			typ := x.typ
 			def.setUnderlying(typ)
 			return typ
 		case invalid:
 			// ignore - error reported before
 		case novalue:
 			check.errorf(&x, _NotAType, "%s used as type", &x)
 		default:
 			check.errorf(&x, _NotAType, "%s is not a type", &x)
 		}

 	case *ast.IndexExpr, *ast.IndexListExpr:
 		ix := typeparams.UnpackIndexExpr(e)
 		if !check.allowVersion(check.pkg, 1, 18) {
 			check.softErrorf(inNode(e, ix.Lbrack), _Todo, "type instantiation requires go1.18 or later")
 		}
 		// TODO(rfindley): type instantiation should require go1.18
 		return check.instantiatedType(ix.X, ix.Indices, def)

 	case *ast.ParenExpr:
 		// Generic types must be instantiated before they can be used in any form.
 		// Consequently, generic types cannot be parenthesized.
 		return check.definedType(e.X, def)

 	case *ast.ArrayType:
 		if e.Len != nil {
 			typ := new(Array)
 			def.setUnderlying(typ)
 			typ.len = check.arrayLength(e.Len)
 			typ.elem = check.varType(e.Elt)
 			return typ
 		}

 		typ := new(Slice)
 		def.setUnderlying(typ)
 		typ.elem = check.varType(e.Elt)
 		return typ

 	case *ast.Ellipsis:
 		// dots are handled explicitly where they are legal
 		// (array composite literals and parameter lists)
 		check.error(e, _InvalidDotDotDot, "invalid use of '...'")
 		check.use(e.Elt)

 	case *ast.StructType:
 		typ := new(Struct)
 		def.setUnderlying(typ)
 		check.structType(typ, e)
 		return typ

 	case *ast.StarExpr:
 		typ := new(Pointer)
 		def.setUnderlying(typ)
 		typ.base = check.varType(e.X)
 		return typ

 	case *ast.FuncType:
 		typ := new(Signature)
 		def.setUnderlying(typ)
 		check.funcType(typ, nil, e)
 		return typ

 	case *ast.InterfaceType:
 		typ := new(Interface)
 		def.setUnderlying(typ)
 		if def != nil {
 			typ.obj = def.obj
 		}
 		check.interfaceType(typ, e, def)
 		return typ

 	case *ast.MapType:
 		typ := new(Map)
 		def.setUnderlying(typ)

 		typ.key = check.varType(e.Key)
 		typ.elem = check.varType(e.Value)

 		// spec: "The comparison operators == and != must be fully defined
 		// for operands of the key type; thus the key type must not be a
 		// function, map, or slice."
 		//
 		// Delay this check because it requires fully setup types;
 		// it is safe to continue in any case (was issue 6667).
 		check.later(func() {
 			if !Comparable(typ.key) {
 				var why string
 				if asTypeParam(typ.key) != nil {
 					why = " (missing comparable constraint)"
 				}
 				check.errorf(e.Key, _IncomparableMapKey, "incomparable map key type %s%s", typ.key, why)
 			}
 		})

 		return typ

 	case *ast.ChanType:
 		typ := new(Chan)
 		def.setUnderlying(typ)

 		dir := SendRecv
 		switch e.Dir {
 		case ast.SEND | ast.RECV:
 			// nothing to do
 		case ast.SEND:
 			dir = SendOnly
 		case ast.RECV:
 			dir = RecvOnly
 		default:
 			check.invalidAST(e, "unknown channel direction %d", e.Dir)
 			// ok to continue
 		}

 		typ.dir = dir
 		typ.elem = check.varType(e.Value)
 		return typ

 	default:
 		check.errorf(e0, _NotAType, "%s is not a type", e0)
 	}

 	typ := Typ[Invalid]
 	def.setUnderlying(typ)
 	return typ
 }

 func (check *Checker) instantiatedType(x ast.Expr, targsx []ast.Expr, def *Named) Type {
 	gtyp := check.genericType(x, true)
 	if gtyp == Typ[Invalid] {
 		return gtyp // error already reported
 	}
 	base, _ := gtyp.(*Named)
 	if base == nil {
 		panic(fmt.Sprintf("%v: cannot instantiate %v", x.Pos(), gtyp))
 	}

 	// evaluate arguments
 	targs := check.typeList(targsx)
 	if targs == nil {
 		def.setUnderlying(Typ[Invalid]) // avoid later errors due to lazy instantiation
 		return Typ[Invalid]
 	}

 	// determine argument positions
 	posList := make([]token.Pos, len(targs))
 	for i, arg := range targsx {
 		posList[i] = arg.Pos()
 	}

 	typ := check.instantiate(x.Pos(), base, targs, posList)
 	def.setUnderlying(typ)

 	// make sure we check instantiation works at least once
 	// and that the resulting type is valid
 	check.later(func() {
 		check.validType(typ, nil)
 	})

 	return typ
 }

 // arrayLength type-checks the array length expression e
 // and returns the constant length >= 0, or a value < 0
 // to indicate an error (and thus an unknown length).
 func (check *Checker) arrayLength(e ast.Expr) int64 {
 	// If e is an undeclared identifier, the array declaration might be an
 	// attempt at a parameterized type declaration with missing constraint.
 	// Provide a better error message than just "undeclared name: X".
 	if name, _ := e.(*ast.Ident); name != nil && check.lookup(name.Name) == nil {
 		check.errorf(name, _InvalidArrayLen, "undeclared name %s for array length", name.Name)
 		return -1
 	}

 	var x operand
 	check.expr(&x, e)
 	if x.mode != constant_ {
 		if x.mode != invalid {
 			check.errorf(&x, _InvalidArrayLen, "array length %s must be constant", &x)
 		}
 		return -1
 	}

 	if isUntyped(x.typ) || isInteger(x.typ) {
 		if val := constant.ToInt(x.val); val.Kind() == constant.Int {
 			if representableConst(val, check, Typ[Int], nil) {
 				if n, ok := constant.Int64Val(val); ok && n >= 0 {
 					return n
 				}
 				check.errorf(&x, _InvalidArrayLen, "invalid array length %s", &x)
 				return -1
 			}
 		}
 	}

 	check.errorf(&x, _InvalidArrayLen, "array length %s must be integer", &x)
 	return -1
 }

 // typeList provides the list of types corresponding to the incoming expression list.
 // If an error occurred, the result is nil, but all list elements were type-checked.
 func (check *Checker) typeList(list []ast.Expr) []Type {
 	res := make([]Type, len(list)) // res != nil even if len(list) == 0
 	for i, x := range list {
 		t := check.varType(x)
 		if t == Typ[Invalid] {
 			res = nil
 		}
 		if res != nil {
 			res[i] = t
 		}
 	}
 	return res
 }
	// Copyright 2013 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// This file implements type-checking of identifiers and type expressions.

	package types

	import (
	"fmt"
	"go/ast"
	"go/constant"
	"go/internal/typeparams"
	"go/token"
	"strings"
	)

	// ident type-checks identifier e and initializes x with the value or type of e.
	// If an error occurred, x.mode is set to invalid.
	// For the meaning of def, see Checker.definedType, below.
	// If wantType is set, the identifier e is expected to denote a type.
	//
	func (check Checker) ident(x operand, e ast.Ident, def Named, wantType bool) {
	x.mode = invalid
	x.expr = e

	// Note that we cannot use check.lookup here because the returned scope
	// may be different from obj.Parent(). See also Scope.LookupParent doc.
	scope, obj := check.scope.LookupParent(e.Name, check.pos)
	switch obj {
	case nil:
	if e.Name == "_" {
	check.error(e, _InvalidBlank, "cannot use _ as value or type")
	} else {
	check.errorf(e, _UndeclaredName, "undeclared name: %s", e.Name)
	}
	return
	case universeAny, universeComparable:
	// complain if necessary
	if !check.allowVersion(check.pkg, 1, 18) {
	check.errorf(e, _UndeclaredName, "undeclared name: %s (requires version go1.18 or later)", e.Name)
	return // avoid follow-on errors
	}
	if obj == universeAny {
	// If we allow "any" for general use, this if-statement can be removed (issue #33232).
	check.softErrorf(e, _Todo, "cannot use any outside constraint position")
	// ok to continue
	}
	}
	check.recordUse(e, obj)

	// Type-check the object.
	// Only call Checker.objDecl if the object doesn't have a type yet
	// (in which case we must actually determine it) or the object is a
	// TypeName and we also want a type (in which case we might detect
	// a cycle which needs to be reported). Otherwise we can skip the
	// call and avoid a possible cycle error in favor of the more
	// informative "not a type/value" error that this function's caller
	// will issue (see issue #25790).
	typ := obj.Type()
	if _, gotType := obj.(*TypeName); typ == nil \|\| gotType && wantType {
	check.objDecl(obj, def)
	typ = obj.Type() // type must have been assigned by Checker.objDecl
	}
	assert(typ != nil)

	// The object may have been dot-imported.
	// If so, mark the respective package as used.
	// (This code is only needed for dot-imports. Without them,
	// we only have to mark variables, see *Var case below).
	if pkgName := check.dotImportMap[dotImportKey{scope, obj.Name()}]; pkgName != nil {
	pkgName.used = true
	}

	switch obj := obj.(type) {
	case *PkgName:
	check.errorf(e, _InvalidPkgUse, "use of package %s not in selector", obj.name)
	return

	case *Const:
	check.addDeclDep(obj)
	if typ == Typ[Invalid] {
	return
	}
	if obj == universeIota {
	if check.iota == nil {
	check.errorf(e, _InvalidIota, "cannot use iota outside constant declaration")
	return
	}
	x.val = check.iota
	} else {
	x.val = obj.val
	}
	assert(x.val != nil)
	x.mode = constant_

	case *TypeName:
	x.mode = typexpr

	case *Var:
	// It's ok to mark non-local variables, but ignore variables
	// from other packages to avoid potential race conditions with
	// dot-imported variables.
	if obj.pkg == check.pkg {
	obj.used = true
	}
	check.addDeclDep(obj)
	if typ == Typ[Invalid] {
	return
	}
	x.mode = variable

	case *Func:
	check.addDeclDep(obj)
	x.mode = value

	case *Builtin:
	x.id = obj.id
	x.mode = builtin

	case *Nil:
	x.mode = value

	default:
	unreachable()
	}

	x.typ = typ
	}

	// typ type-checks the type expression e and returns its type, or Typ[Invalid].
	// The type must not be an (uninstantiated) generic type.
	func (check *Checker) typ(e ast.Expr) Type {
	return check.definedType(e, nil)
	}

	// varType type-checks the type expression e and returns its type, or Typ[Invalid].
	// The type must not be an (uninstantiated) generic type and it must not be a
	// constraint interface.
	func (check *Checker) varType(e ast.Expr) Type {
	typ := check.definedType(e, nil)
	// We don't want to call under() (via asInterface) or complete interfaces while we
	// are in the middle of type-checking parameter declarations that might belong to
	// interface methods. Delay this check to the end of type-checking.
	check.later(func() {
	if t := asInterface(typ); t != nil {
	tset := computeInterfaceTypeSet(check, e.Pos(), t) // TODO(gri) is this the correct position?
	if tset.IsConstraint() {
	if tset.comparable {
	check.softErrorf(e, _Todo, "interface is (or embeds) comparable")
	} else {
	check.softErrorf(e, _Todo, "interface contains type constraints")
	}
	}
	}
	})

	return typ
	}

	// definedType is like typ but also accepts a type name def.
	// If def != nil, e is the type specification for the defined type def, declared
	// in a type declaration, and def.underlying will be set to the type of e before
	// any components of e are type-checked.
	//
	func (check Checker) definedType(e ast.Expr, def Named) Type {
	typ := check.typInternal(e, def)
	assert(isTyped(typ))
	if isGeneric(typ) {
	check.errorf(e, _Todo, "cannot use generic type %s without instantiation", typ)
	typ = Typ[Invalid]
	}
	check.recordTypeAndValue(e, typexpr, typ, nil)
	return typ
	}

	// genericType is like typ but the type must be an (uninstantiated) generic type.
	func (check *Checker) genericType(e ast.Expr, reportErr bool) Type {
	typ := check.typInternal(e, nil)
	assert(isTyped(typ))
	if typ != Typ[Invalid] && !isGeneric(typ) {
	if reportErr {
	check.errorf(e, _Todo, "%s is not a generic type", typ)
	}
	typ = Typ[Invalid]
	}
	// TODO(gri) what is the correct call below?
	check.recordTypeAndValue(e, typexpr, typ, nil)
	return typ
	}

	// goTypeName returns the Go type name for typ and
	// removes any occurrences of "types." from that name.
	func goTypeName(typ Type) string {
	return strings.ReplaceAll(fmt.Sprintf("%T", typ), "types.", "")
	}

	// typInternal drives type checking of types.
	// Must only be called by definedType or genericType.
	//
	func (check Checker) typInternal(e0 ast.Expr, def Named) (T Type) {
	if trace {
	check.trace(e0.Pos(), "type %s", e0)
	check.indent++
	defer func() {
	check.indent--
	var under Type
	if T != nil {
	// Calling under() here may lead to endless instantiations.
	// Test case: type T[P any] *T[P]
	// TODO(gri) investigate if that's a bug or to be expected
	// (see also analogous comment in Checker.instantiate).
	under = safeUnderlying(T)
	}
	if T == under {
	check.trace(e0.Pos(), "=> %s // %s", T, goTypeName(T))
	} else {
	check.trace(e0.Pos(), "=> %s (under = %s) // %s", T, under, goTypeName(T))
	}
	}()
	}

	switch e := e0.(type) {
	case *ast.BadExpr:
	// ignore - error reported before

	case *ast.Ident:
	var x operand
	check.ident(&x, e, def, true)

	switch x.mode {
	case typexpr:
	typ := x.typ
	def.setUnderlying(typ)
	return typ
	case invalid:
	// ignore - error reported before
	case novalue:
	check.errorf(&x, _NotAType, "%s used as type", &x)
	default:
	check.errorf(&x, _NotAType, "%s is not a type", &x)
	}

	case *ast.SelectorExpr:
	var x operand
	check.selector(&x, e)

	switch x.mode {
	case typexpr:
	typ := x.typ
	def.setUnderlying(typ)
	return typ
	case invalid:
	// ignore - error reported before
	case novalue:
	check.errorf(&x, _NotAType, "%s used as type", &x)
	default:
	check.errorf(&x, _NotAType, "%s is not a type", &x)
	}

	case ast.IndexExpr, ast.IndexListExpr:
	ix := typeparams.UnpackIndexExpr(e)
	if !check.allowVersion(check.pkg, 1, 18) {
	check.softErrorf(inNode(e, ix.Lbrack), _Todo, "type instantiation requires go1.18 or later")
	}
	// TODO(rfindley): type instantiation should require go1.18
	return check.instantiatedType(ix.X, ix.Indices, def)

	case *ast.ParenExpr:
	// Generic types must be instantiated before they can be used in any form.
	// Consequently, generic types cannot be parenthesized.
	return check.definedType(e.X, def)

	case *ast.ArrayType:
	if e.Len != nil {
	typ := new(Array)
	def.setUnderlying(typ)
	typ.len = check.arrayLength(e.Len)
	typ.elem = check.varType(e.Elt)
	return typ
	}

	typ := new(Slice)
	def.setUnderlying(typ)
	typ.elem = check.varType(e.Elt)
	return typ

	case *ast.Ellipsis:
	// dots are handled explicitly where they are legal
	// (array composite literals and parameter lists)
	check.error(e, _InvalidDotDotDot, "invalid use of '...'")
	check.use(e.Elt)

	case *ast.StructType:
	typ := new(Struct)
	def.setUnderlying(typ)
	check.structType(typ, e)
	return typ

	case *ast.StarExpr:
	typ := new(Pointer)
	def.setUnderlying(typ)
	typ.base = check.varType(e.X)
	return typ

	case *ast.FuncType:
	typ := new(Signature)
	def.setUnderlying(typ)
	check.funcType(typ, nil, e)
	return typ

	case *ast.InterfaceType:
	typ := new(Interface)
	def.setUnderlying(typ)
	if def != nil {
	typ.obj = def.obj
	}
	check.interfaceType(typ, e, def)
	return typ

	case *ast.MapType:
	typ := new(Map)
	def.setUnderlying(typ)

	typ.key = check.varType(e.Key)
	typ.elem = check.varType(e.Value)

	// spec: "The comparison operators == and != must be fully defined
	// for operands of the key type; thus the key type must not be a
	// function, map, or slice."
	//
	// Delay this check because it requires fully setup types;
	// it is safe to continue in any case (was issue 6667).
	check.later(func() {
	if !Comparable(typ.key) {
	var why string
	if asTypeParam(typ.key) != nil {
	why = " (missing comparable constraint)"
	}
	check.errorf(e.Key, _IncomparableMapKey, "incomparable map key type %s%s", typ.key, why)
	}
	})

	return typ

	case *ast.ChanType:
	typ := new(Chan)
	def.setUnderlying(typ)

	dir := SendRecv
	switch e.Dir {
	case ast.SEND \| ast.RECV:
	// nothing to do
	case ast.SEND:
	dir = SendOnly
	case ast.RECV:
	dir = RecvOnly
	default:
	check.invalidAST(e, "unknown channel direction %d", e.Dir)
	// ok to continue
	}

	typ.dir = dir
	typ.elem = check.varType(e.Value)
	return typ

	default:
	check.errorf(e0, _NotAType, "%s is not a type", e0)
	}

	typ := Typ[Invalid]
	def.setUnderlying(typ)
	return typ
	}

	func (check Checker) instantiatedType(x ast.Expr, targsx []ast.Expr, def Named) Type {
	gtyp := check.genericType(x, true)
	if gtyp == Typ[Invalid] {
	return gtyp // error already reported
	}
	base, _ := gtyp.(*Named)
	if base == nil {
	panic(fmt.Sprintf("%v: cannot instantiate %v", x.Pos(), gtyp))
	}

	// evaluate arguments
	targs := check.typeList(targsx)
	if targs == nil {
	def.setUnderlying(Typ[Invalid]) // avoid later errors due to lazy instantiation
	return Typ[Invalid]
	}

	// determine argument positions
	posList := make([]token.Pos, len(targs))
	for i, arg := range targsx {
	posList[i] = arg.Pos()
	}

	typ := check.instantiate(x.Pos(), base, targs, posList)
	def.setUnderlying(typ)

	// make sure we check instantiation works at least once
	// and that the resulting type is valid
	check.later(func() {
	check.validType(typ, nil)
	})

	return typ
	}

	// arrayLength type-checks the array length expression e
	// and returns the constant length >= 0, or a value < 0
	// to indicate an error (and thus an unknown length).
	func (check *Checker) arrayLength(e ast.Expr) int64 {
	// If e is an undeclared identifier, the array declaration might be an
	// attempt at a parameterized type declaration with missing constraint.
	// Provide a better error message than just "undeclared name: X".
	if name, _ := e.(*ast.Ident); name != nil && check.lookup(name.Name) == nil {
	check.errorf(name, _InvalidArrayLen, "undeclared name %s for array length", name.Name)
	return -1
	}

	var x operand
	check.expr(&x, e)
	if x.mode != constant_ {
	if x.mode != invalid {
	check.errorf(&x, _InvalidArrayLen, "array length %s must be constant", &x)
	}
	return -1
	}

	if isUntyped(x.typ) \|\| isInteger(x.typ) {
	if val := constant.ToInt(x.val); val.Kind() == constant.Int {
	if representableConst(val, check, Typ[Int], nil) {
	if n, ok := constant.Int64Val(val); ok && n >= 0 {
	return n
	}
	check.errorf(&x, _InvalidArrayLen, "invalid array length %s", &x)
	return -1
	}
	}
	}

	check.errorf(&x, _InvalidArrayLen, "array length %s must be integer", &x)
	return -1
	}

	// typeList provides the list of types corresponding to the incoming expression list.
	// If an error occurred, the result is nil, but all list elements were type-checked.
	func (check *Checker) typeList(list []ast.Expr) []Type {
	res := make([]Type, len(list)) // res != nil even if len(list) == 0
	for i, x := range list {
	t := check.varType(x)
	if t == Typ[Invalid] {
	res = nil
	}
	if res != nil {
	res[i] = t
	}
	}
	return res
	}