src/go/types/typexpr.go

// 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"
	"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 == "_" {
			// Blank identifiers are never declared, but the current identifier may
			// be a placeholder for a receiver type parameter. In this case we can
			// resolve its type and object from Checker.recvTParamMap.
			if tpar := check.recvTParamMap[e]; tpar != nil {
				x.mode = typexpr
				x.typ = tpar
			} else {
				check.error(e, _InvalidBlank, "cannot use _ as value or type")
			}
		} else {
			check.errorf(e, _UndeclaredName, "undeclared name: %s", e.Name)
		}
		return
	case universeAny, universeComparable:
		if !check.allowVersion(check.pkg, 1, 18) {
			check.versionErrorf(e, _UndeclaredName, "go1.18", "predeclared %s", e.Name)
			return // avoid follow-on errors
		}
	}
	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:
		if check.isBrokenAlias(obj) {
			check.errorf(e, _InvalidDeclCycle, "invalid use of type alias %s in recursive type (see issue #50729)", obj.name)
			return
		}
		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)
	check.validVarType(e, typ)
	return typ
}

// validVarType reports an error if typ is a constraint interface.
// The expression e is used for error reporting, if any.
func (check *Checker) validVarType(e ast.Expr, typ Type) {
	// If we have a type parameter there's nothing to do.
	if isTypeParam(typ) {
		return
	}

	// We don't want to call under() 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, _ := under(typ).(*Interface); t != nil {
			tset := computeInterfaceTypeSet(check, e.Pos(), t) // TODO(gri) is this the correct position?
			if !tset.IsMethodSet() {
				if tset.comparable {
					check.softErrorf(e, _MisplacedConstraintIface, "interface is (or embeds) comparable")
				} else {
					check.softErrorf(e, _MisplacedConstraintIface, "interface contains type constraints")
				}
			}
		}
	}).describef(e, "check var type %s", 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, _WrongTypeArgCount, "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. If reason is non-nil and the type expression was a valid type but not
// generic, reason will be populated with a message describing the error.
func (check *Checker) genericType(e ast.Expr, reason *string) Type {
	typ := check.typInternal(e, nil)
	assert(isTyped(typ))
	if typ != Typ[Invalid] && !isGeneric(typ) {
		if reason != nil {
			*reason = check.sprintf("%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]
				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, def)

		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), _UnsupportedFeature, "type instantiation requires go1.18 or later")
		}
		return check.instantiatedType(ix, 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(Slice)
			def.setUnderlying(typ)
			typ.elem = check.varType(e.Elt)
			return typ
		}

		typ := new(Array)
		def.setUnderlying(typ)
		typ.len = check.arrayLength(e.Len)
		typ.elem = check.varType(e.Elt)
		if typ.len >= 0 {
			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)
		typ.base = Typ[Invalid] // avoid nil base in invalid recursive type declaration
		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 := check.newInterface()
		def.setUnderlying(typ)
		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 isTypeParam(typ.key) {
					why = " (missing comparable constraint)"
				}
				check.errorf(e.Key, _IncomparableMapKey, "incomparable map key type %s%s", typ.key, why)
			}
		}).describef(e.Key, "check map key %s", typ.key)

		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(ix *typeparams.IndexExpr, def *Named) (res Type) {
	pos := ix.X.Pos()
	if trace {
		check.trace(pos, "-- instantiating type %s with %s", ix.X, ix.Indices)
		check.indent++
		defer func() {
			check.indent--
			// Don't format the underlying here. It will always be nil.
			check.trace(pos, "=> %s", res)
		}()
	}

	var reason string
	gtyp := check.genericType(ix.X, &reason)
	if reason != "" {
		check.invalidOp(ix.Orig, _NotAGenericType, "%s (%s)", ix.Orig, reason)
	}
	if gtyp == Typ[Invalid] {
		return gtyp // error already reported
	}

	orig, _ := gtyp.(*Named)
	if orig == nil {
		panic(fmt.Sprintf("%v: cannot instantiate %v", ix.Pos(), gtyp))
	}

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

	// enableTypeTypeInference controls whether to infer missing type arguments
	// using constraint type inference. See issue #51527.
	const enableTypeTypeInference = false

	// create the instance
	ctxt := check.bestContext(nil)
	h := ctxt.instanceHash(orig, targs)
	// targs may be incomplete, and require inference. In any case we should de-duplicate.
	inst, _ := ctxt.lookup(h, orig, targs).(*Named)
	// If inst is non-nil, we can't just return here. Inst may have been
	// constructed via recursive substitution, in which case we wouldn't do the
	// validation below. Ensure that the validation (and resulting errors) runs
	// for each instantiated type in the source.
	if inst == nil {
		// x may be a selector for an imported type; use its start pos rather than x.Pos().
		tname := NewTypeName(ix.Pos(), orig.obj.pkg, orig.obj.name, nil)
		inst = check.newNamed(tname, orig, nil, nil) // underlying, methods and tparams are set when named is resolved
		inst.targs = newTypeList(targs)
		inst = ctxt.update(h, orig, targs, inst).(*Named)
	}
	def.setUnderlying(inst)

	inst.resolver = func(ctxt *Context, n *Named) (*TypeParamList, Type, *methodList) {
		tparams := n.orig.TypeParams().list()

		targs := n.targs.list()
		if enableTypeTypeInference && len(targs) < len(tparams) {
			// If inference fails, len(inferred) will be 0, and inst.underlying will
			// be set to Typ[Invalid] in expandNamed.
			inferred := check.infer(ix.Orig, tparams, targs, nil, nil)
			if len(inferred) > len(targs) {
				n.targs = newTypeList(inferred)
			}
		}

		return expandNamed(ctxt, n, pos)
	}

	// orig.tparams may not be set up, so we need to do expansion later.
	check.later(func() {
		// This is an instance from the source, not from recursive substitution,
		// and so it must be resolved during type-checking so that we can report
		// errors.
		inst.resolve(ctxt)
		// Since check is non-nil, we can still mutate inst. Unpinning the resolver
		// frees some memory.
		inst.resolver = nil
		check.recordInstance(ix.Orig, inst.TypeArgs().list(), inst)

		if check.validateTArgLen(pos, inst.tparams.Len(), inst.targs.Len()) {
			if i, err := check.verify(pos, inst.tparams.list(), inst.targs.list()); err != nil {
				// best position for error reporting
				pos := ix.Pos()
				if i < len(ix.Indices) {
					pos = ix.Indices[i].Pos()
				}
				check.softErrorf(atPos(pos), _InvalidTypeArg, err.Error())
			} else {
				check.mono.recordInstance(check.pkg, pos, inst.tparams.list(), inst.targs.list(), ix.Indices)
			}
		}

		check.validType(inst)
	}).describef(ix, "resolve instance %s", inst)

	return inst
}

// 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 identifier, the array declaration might be an
	// attempt at a parameterized type declaration with missing
	// constraint. Provide an error message that mentions array
	// length.
	if name, _ := e.(*ast.Ident); name != nil {
		obj := check.lookup(name.Name)
		if obj == nil {
			check.errorf(name, _InvalidArrayLen, "undeclared name %s for array length", name.Name)
			return -1
		}
		if _, ok := obj.(*Const); !ok {
			check.errorf(name, _InvalidArrayLen, "invalid array length %s", 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
}