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

 // Copyright 2012 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 typechecking of conversions.

 package types

 import (
 	"go/constant"
 	"go/token"
 	"unicode"
 )

 // Conversion type-checks the conversion T(x).
 // The result is in x.
 func (check *Checker) conversion(x *operand, T Type) {
 	constArg := x.mode == constant_

 	constConvertibleTo := func(T Type, val *constant.Value) bool {
 		switch t, _ := under(T).(*Basic); {
 		case t == nil:
 			// nothing to do
 		case representableConst(x.val, check, t, val):
 			return true
 		case isInteger(x.typ) && isString(t):
 			codepoint := unicode.ReplacementChar
 			if i, ok := constant.Uint64Val(x.val); ok && i <= unicode.MaxRune {
 				codepoint = rune(i)
 			}
 			if val != nil {
 				*val = constant.MakeString(string(codepoint))
 			}
 			return true
 		}
 		return false
 	}

 	var ok bool
 	var cause string
 	switch {
 	case constArg && isConstType(T):
 		// constant conversion
 		ok = constConvertibleTo(T, &x.val)
 	case constArg && isTypeParam(T):
 		// x is convertible to T if it is convertible
 		// to each specific type in the type set of T.
 		// If T's type set is empty, or if it doesn't
 		// have specific types, constant x cannot be
 		// converted.
 		ok = T.(*TypeParam).underIs(func(u Type) bool {
 			// u is nil if there are no specific type terms
 			if u == nil {
 				cause = check.sprintf("%s does not contain specific types", T)
 				return false
 			}
 			if isString(x.typ) && isBytesOrRunes(u) {
 				return true
 			}
 			if !constConvertibleTo(u, nil) {
 				cause = check.sprintf("cannot convert %s to %s (in %s)", x, u, T)
 				return false
 			}
 			return true
 		})
 		x.mode = value // type parameters are not constants
 	case x.convertibleTo(check, T, &cause):
 		// non-constant conversion
 		ok = true
 		x.mode = value
 	}

 	if !ok {
 		// TODO(rfindley): use types2-style error reporting here.
 		if compilerErrorMessages {
 			if cause != "" {
 				// Add colon at end of line if we have a following cause.
 				err := newErrorf(x, _InvalidConversion, "cannot convert %s to type %s:", x, T)
 				err.errorf(token.NoPos, cause)
 				check.report(err)
 			} else {
 				check.errorf(x, _InvalidConversion, "cannot convert %s to type %s", x, T)
 			}
 		} else {
 			if cause != "" {
 				check.errorf(x, _InvalidConversion, "cannot convert %s to %s (%s)", x, T, cause)
 			} else {
 				check.errorf(x, _InvalidConversion, "cannot convert %s to %s", x, T)
 			}
 		}
 		x.mode = invalid
 		return
 	}

 	// The conversion argument types are final. For untyped values the
 	// conversion provides the type, per the spec: "A constant may be
 	// given a type explicitly by a constant declaration or conversion,...".
 	if isUntyped(x.typ) {
 		final := T
 		// - For conversions to interfaces, use the argument's default type.
 		// - For conversions of untyped constants to non-constant types, also
 		//   use the default type (e.g., []byte("foo") should report string
 		//   not []byte as type for the constant "foo").
 		// - Keep untyped nil for untyped nil arguments.
 		// - For constant integer to string conversions, keep the argument type.
 		//   (See also the TODO below.)
 		if isNonTypeParamInterface(T) || constArg && !isConstType(T) || x.isNil() {
 			final = Default(x.typ) // default type of untyped nil is untyped nil
 		} else if x.mode == constant_ && isInteger(x.typ) && allString(T) {
 			final = x.typ
 		}
 		check.updateExprType(x.expr, final, true)
 	}

 	x.typ = T
 }

 // TODO(gri) convertibleTo checks if T(x) is valid. It assumes that the type
 // of x is fully known, but that's not the case for say string(1<<s + 1.0):
 // Here, the type of 1<<s + 1.0 will be UntypedFloat which will lead to the
 // (correct!) refusal of the conversion. But the reported error is essentially
 // "cannot convert untyped float value to string", yet the correct error (per
 // the spec) is that we cannot shift a floating-point value: 1 in 1<<s should
 // be converted to UntypedFloat because of the addition of 1.0. Fixing this
 // is tricky because we'd have to run updateExprType on the argument first.
 // (Issue #21982.)

 // convertibleTo reports whether T(x) is valid. In the failure case, *cause
 // may be set to the cause for the failure.
 // The check parameter may be nil if convertibleTo is invoked through an
 // exported API call, i.e., when all methods have been type-checked.
 func (x *operand) convertibleTo(check *Checker, T Type, cause *string) bool {
 	// "x is assignable to T"
 	if ok, _ := x.assignableTo(check, T, cause); ok {
 		return true
 	}

 	// "V and T have identical underlying types if tags are ignored
 	// and V and T are not type parameters"
 	V := x.typ
 	Vu := under(V)
 	Tu := under(T)
 	Vp, _ := V.(*TypeParam)
 	Tp, _ := T.(*TypeParam)
 	if IdenticalIgnoreTags(Vu, Tu) && Vp == nil && Tp == nil {
 		return true
 	}

 	// "V and T are unnamed pointer types and their pointer base types
 	// have identical underlying types if tags are ignored
 	// and their pointer base types are not type parameters"
 	if V, ok := V.(*Pointer); ok {
 		if T, ok := T.(*Pointer); ok {
 			if IdenticalIgnoreTags(under(V.base), under(T.base)) && !isTypeParam(V.base) && !isTypeParam(T.base) {
 				return true
 			}
 		}
 	}

 	// "V and T are both integer or floating point types"
 	if isIntegerOrFloat(Vu) && isIntegerOrFloat(Tu) {
 		return true
 	}

 	// "V and T are both complex types"
 	if isComplex(Vu) && isComplex(Tu) {
 		return true
 	}

 	// "V is an integer or a slice of bytes or runes and T is a string type"
 	if (isInteger(Vu) || isBytesOrRunes(Vu)) && isString(Tu) {
 		return true
 	}

 	// "V is a string and T is a slice of bytes or runes"
 	if isString(Vu) && isBytesOrRunes(Tu) {
 		return true
 	}

 	// package unsafe:
 	// "any pointer or value of underlying type uintptr can be converted into a unsafe.Pointer"
 	if (isPointer(Vu) || isUintptr(Vu)) && isUnsafePointer(Tu) {
 		return true
 	}
 	// "and vice versa"
 	if isUnsafePointer(Vu) && (isPointer(Tu) || isUintptr(Tu)) {
 		return true
 	}

 	// "V a slice, T is a pointer-to-array type,
 	// and the slice and array types have identical element types."
 	if s, _ := Vu.(*Slice); s != nil {
 		if p, _ := Tu.(*Pointer); p != nil {
 			if a, _ := under(p.Elem()).(*Array); a != nil {
 				if Identical(s.Elem(), a.Elem()) {
 					if check == nil || check.allowVersion(check.pkg, 1, 17) {
 						return true
 					}
 					if cause != nil {
 						*cause = "conversion of slices to array pointers requires go1.17 or later"
 					}
 				}
 			}
 		}
 	}

 	// optimization: if we don't have type parameters, we're done
 	if Vp == nil && Tp == nil {
 		return false
 	}

 	errorf := func(format string, args ...any) {
 		if check != nil && cause != nil {
 			msg := check.sprintf(format, args...)
 			if *cause != "" {
 				msg += "\n\t" + *cause
 			}
 			*cause = msg
 		}
 	}

 	// generic cases with specific type terms
 	// (generic operands cannot be constants, so we can ignore x.val)
 	switch {
 	case Vp != nil && Tp != nil:
 		x := *x // don't clobber outer x
 		return Vp.is(func(V *term) bool {
 			if V == nil {
 				return false // no specific types
 			}
 			x.typ = V.typ
 			return Tp.is(func(T *term) bool {
 				if T == nil {
 					return false // no specific types
 				}
 				if !x.convertibleTo(check, T.typ, cause) {
 					errorf("cannot convert %s (in %s) to %s (in %s)", V.typ, Vp, T.typ, Tp)
 					return false
 				}
 				return true
 			})
 		})
 	case Vp != nil:
 		x := *x // don't clobber outer x
 		return Vp.is(func(V *term) bool {
 			if V == nil {
 				return false // no specific types
 			}
 			x.typ = V.typ
 			if !x.convertibleTo(check, T, cause) {
 				errorf("cannot convert %s (in %s) to %s", V.typ, Vp, T)
 				return false
 			}
 			return true
 		})
 	case Tp != nil:
 		return Tp.is(func(T *term) bool {
 			if T == nil {
 				return false // no specific types
 			}
 			if !x.convertibleTo(check, T.typ, cause) {
 				errorf("cannot convert %s to %s (in %s)", x.typ, T.typ, Tp)
 				return false
 			}
 			return true
 		})
 	}

 	return false
 }

 func isUintptr(typ Type) bool {
 	t, _ := under(typ).(*Basic)
 	return t != nil && t.kind == Uintptr
 }

 func isUnsafePointer(typ Type) bool {
 	t, _ := under(typ).(*Basic)
 	return t != nil && t.kind == UnsafePointer
 }

 func isPointer(typ Type) bool {
 	_, ok := under(typ).(*Pointer)
 	return ok
 }

 func isBytesOrRunes(typ Type) bool {
 	if s, _ := under(typ).(*Slice); s != nil {
 		t, _ := under(s.elem).(*Basic)
 		return t != nil && (t.kind == Byte || t.kind == Rune)
 	}
 	return false
 }
	// Copyright 2012 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 typechecking of conversions.

	package types

	import (
	"go/constant"
	"go/token"
	"unicode"
	)

	// Conversion type-checks the conversion T(x).
	// The result is in x.
	func (check Checker) conversion(x operand, T Type) {
	constArg := x.mode == constant_

	constConvertibleTo := func(T Type, val *constant.Value) bool {
	switch t, _ := under(T).(*Basic); {
	case t == nil:
	// nothing to do
	case representableConst(x.val, check, t, val):
	return true
	case isInteger(x.typ) && isString(t):
	codepoint := unicode.ReplacementChar
	if i, ok := constant.Uint64Val(x.val); ok && i <= unicode.MaxRune {
	codepoint = rune(i)
	}
	if val != nil {
	*val = constant.MakeString(string(codepoint))
	}
	return true
	}
	return false
	}

	var ok bool
	var cause string
	switch {
	case constArg && isConstType(T):
	// constant conversion
	ok = constConvertibleTo(T, &x.val)
	case constArg && isTypeParam(T):
	// x is convertible to T if it is convertible
	// to each specific type in the type set of T.
	// If T's type set is empty, or if it doesn't
	// have specific types, constant x cannot be
	// converted.
	ok = T.(*TypeParam).underIs(func(u Type) bool {
	// u is nil if there are no specific type terms
	if u == nil {
	cause = check.sprintf("%s does not contain specific types", T)
	return false
	}
	if isString(x.typ) && isBytesOrRunes(u) {
	return true
	}
	if !constConvertibleTo(u, nil) {
	cause = check.sprintf("cannot convert %s to %s (in %s)", x, u, T)
	return false
	}
	return true
	})
	x.mode = value // type parameters are not constants
	case x.convertibleTo(check, T, &cause):
	// non-constant conversion
	ok = true
	x.mode = value
	}

	if !ok {
	// TODO(rfindley): use types2-style error reporting here.
	if compilerErrorMessages {
	if cause != "" {
	// Add colon at end of line if we have a following cause.
	err := newErrorf(x, _InvalidConversion, "cannot convert %s to type %s:", x, T)
	err.errorf(token.NoPos, cause)
	check.report(err)
	} else {
	check.errorf(x, _InvalidConversion, "cannot convert %s to type %s", x, T)
	}
	} else {
	if cause != "" {
	check.errorf(x, _InvalidConversion, "cannot convert %s to %s (%s)", x, T, cause)
	} else {
	check.errorf(x, _InvalidConversion, "cannot convert %s to %s", x, T)
	}
	}
	x.mode = invalid
	return
	}

	// The conversion argument types are final. For untyped values the
	// conversion provides the type, per the spec: "A constant may be
	// given a type explicitly by a constant declaration or conversion,...".
	if isUntyped(x.typ) {
	final := T
	// - For conversions to interfaces, use the argument's default type.
	// - For conversions of untyped constants to non-constant types, also
	// use the default type (e.g., []byte("foo") should report string
	// not []byte as type for the constant "foo").
	// - Keep untyped nil for untyped nil arguments.
	// - For constant integer to string conversions, keep the argument type.
	// (See also the TODO below.)
	if isNonTypeParamInterface(T) \|\| constArg && !isConstType(T) \|\| x.isNil() {
	final = Default(x.typ) // default type of untyped nil is untyped nil
	} else if x.mode == constant_ && isInteger(x.typ) && allString(T) {
	final = x.typ
	}
	check.updateExprType(x.expr, final, true)
	}

	x.typ = T
	}

	// TODO(gri) convertibleTo checks if T(x) is valid. It assumes that the type
	// of x is fully known, but that's not the case for say string(1<<s + 1.0):
	// Here, the type of 1<<s + 1.0 will be UntypedFloat which will lead to the
	// (correct!) refusal of the conversion. But the reported error is essentially
	// "cannot convert untyped float value to string", yet the correct error (per
	// the spec) is that we cannot shift a floating-point value: 1 in 1<<s should
	// be converted to UntypedFloat because of the addition of 1.0. Fixing this
	// is tricky because we'd have to run updateExprType on the argument first.
	// (Issue #21982.)

	// convertibleTo reports whether T(x) is valid. In the failure case, *cause
	// may be set to the cause for the failure.
	// The check parameter may be nil if convertibleTo is invoked through an
	// exported API call, i.e., when all methods have been type-checked.
	func (x operand) convertibleTo(check Checker, T Type, cause *string) bool {
	// "x is assignable to T"
	if ok, _ := x.assignableTo(check, T, cause); ok {
	return true
	}

	// "V and T have identical underlying types if tags are ignored
	// and V and T are not type parameters"
	V := x.typ
	Vu := under(V)
	Tu := under(T)
	Vp, _ := V.(*TypeParam)
	Tp, _ := T.(*TypeParam)
	if IdenticalIgnoreTags(Vu, Tu) && Vp == nil && Tp == nil {
	return true
	}

	// "V and T are unnamed pointer types and their pointer base types
	// have identical underlying types if tags are ignored
	// and their pointer base types are not type parameters"
	if V, ok := V.(*Pointer); ok {
	if T, ok := T.(*Pointer); ok {
	if IdenticalIgnoreTags(under(V.base), under(T.base)) && !isTypeParam(V.base) && !isTypeParam(T.base) {
	return true
	}
	}
	}

	// "V and T are both integer or floating point types"
	if isIntegerOrFloat(Vu) && isIntegerOrFloat(Tu) {
	return true
	}

	// "V and T are both complex types"
	if isComplex(Vu) && isComplex(Tu) {
	return true
	}

	// "V is an integer or a slice of bytes or runes and T is a string type"
	if (isInteger(Vu) \|\| isBytesOrRunes(Vu)) && isString(Tu) {
	return true
	}

	// "V is a string and T is a slice of bytes or runes"
	if isString(Vu) && isBytesOrRunes(Tu) {
	return true
	}

	// package unsafe:
	// "any pointer or value of underlying type uintptr can be converted into a unsafe.Pointer"
	if (isPointer(Vu) \|\| isUintptr(Vu)) && isUnsafePointer(Tu) {
	return true
	}
	// "and vice versa"
	if isUnsafePointer(Vu) && (isPointer(Tu) \|\| isUintptr(Tu)) {
	return true
	}

	// "V a slice, T is a pointer-to-array type,
	// and the slice and array types have identical element types."
	if s, _ := Vu.(*Slice); s != nil {
	if p, _ := Tu.(*Pointer); p != nil {
	if a, _ := under(p.Elem()).(*Array); a != nil {
	if Identical(s.Elem(), a.Elem()) {
	if check == nil \|\| check.allowVersion(check.pkg, 1, 17) {
	return true
	}
	if cause != nil {
	*cause = "conversion of slices to array pointers requires go1.17 or later"
	}
	}
	}
	}
	}

	// optimization: if we don't have type parameters, we're done
	if Vp == nil && Tp == nil {
	return false
	}

	errorf := func(format string, args ...any) {
	if check != nil && cause != nil {
	msg := check.sprintf(format, args...)
	if *cause != "" {
	msg += "\n\t" + *cause
	}
	*cause = msg
	}
	}

	// generic cases with specific type terms
	// (generic operands cannot be constants, so we can ignore x.val)
	switch {
	case Vp != nil && Tp != nil:
	x := *x // don't clobber outer x
	return Vp.is(func(V *term) bool {
	if V == nil {
	return false // no specific types
	}
	x.typ = V.typ
	return Tp.is(func(T *term) bool {
	if T == nil {
	return false // no specific types
	}
	if !x.convertibleTo(check, T.typ, cause) {
	errorf("cannot convert %s (in %s) to %s (in %s)", V.typ, Vp, T.typ, Tp)
	return false
	}
	return true
	})
	})
	case Vp != nil:
	x := *x // don't clobber outer x
	return Vp.is(func(V *term) bool {
	if V == nil {
	return false // no specific types
	}
	x.typ = V.typ
	if !x.convertibleTo(check, T, cause) {
	errorf("cannot convert %s (in %s) to %s", V.typ, Vp, T)
	return false
	}
	return true
	})
	case Tp != nil:
	return Tp.is(func(T *term) bool {
	if T == nil {
	return false // no specific types
	}
	if !x.convertibleTo(check, T.typ, cause) {
	errorf("cannot convert %s to %s (in %s)", x.typ, T.typ, Tp)
	return false
	}
	return true
	})
	}

	return false
	}

	func isUintptr(typ Type) bool {
	t, _ := under(typ).(*Basic)
	return t != nil && t.kind == Uintptr
	}

	func isUnsafePointer(typ Type) bool {
	t, _ := under(typ).(*Basic)
	return t != nil && t.kind == UnsafePointer
	}

	func isPointer(typ Type) bool {
	_, ok := under(typ).(*Pointer)
	return ok
	}

	func isBytesOrRunes(typ Type) bool {
	if s, _ := under(typ).(*Slice); s != nil {
	t, _ := under(s.elem).(*Basic)
	return t != nil && (t.kind == Byte \|\| t.kind == Rune)
	}
	return false
	}