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conversions.go
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conversions.go
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// 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"
// Conversion type-checks the conversion T(x).
// The result is in x.
func (check *Checker) conversion(x *operand, T Type) {
constArg := x.mode == constant_
var ok bool
switch {
case constArg && isConstType(T):
// constant conversion
switch t := T.Underlying().(*Basic); {
case representableConst(x.val, check.conf, t.kind, &x.val):
ok = true
case isInteger(x.typ) && isString(t):
codepoint := int64(-1)
if i, ok := constant.Int64Val(x.val); ok {
codepoint = i
}
// If codepoint < 0 the absolute value is too large (or unknown) for
// conversion. This is the same as converting any other out-of-range
// value - let string(codepoint) do the work.
x.val = constant.MakeString(string(codepoint))
ok = true
}
case x.convertibleTo(check.conf, T):
// non-constant conversion
x.mode = value
ok = true
}
if !ok {
check.errorf(x.pos(), "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,...".
final := x.typ
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.
if IsInterface(T) || constArg && !isConstType(T) {
final = defaultType(x.typ)
}
check.updateExprType(x.expr, final, true)
}
x.typ = T
}
func (x *operand) convertibleTo(conf *Config, T Type) bool {
// "x is assignable to T"
if x.assignableTo(conf, T, nil) {
return true
}
// "x's type and T have identical underlying types"
V := x.typ
Vu := V.Underlying()
Tu := T.Underlying()
if Identical(Vu, Tu) {
return true
}
// "x's type and T are unnamed pointer types and their pointer base types have identical underlying types"
if V, ok := V.(*Pointer); ok {
if T, ok := T.(*Pointer); ok {
if Identical(V.base.Underlying(), T.base.Underlying()) {
return true
}
}
}
// "x's type and T are both integer or floating point types"
if (isInteger(V) || isFloat(V)) && (isInteger(T) || isFloat(T)) {
return true
}
// "x's type and T are both complex types"
if isComplex(V) && isComplex(T) {
return true
}
// "x is an integer or a slice of bytes or runes and T is a string type"
if (isInteger(V) || isBytesOrRunes(Vu)) && isString(T) {
return true
}
// "x is a string and T is a slice of bytes or runes"
if isString(V) && 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(T) {
return true
}
// "and vice versa"
if isUnsafePointer(V) && (isPointer(Tu) || isUintptr(Tu)) {
return true
}
return false
}
func isUintptr(typ Type) bool {
t, ok := typ.Underlying().(*Basic)
return ok && t.kind == Uintptr
}
func isUnsafePointer(typ Type) bool {
// TODO(gri): Is this (typ.Underlying() instead of just typ) correct?
// The spec does not say so, but gc claims it is. See also
// issue 6326.
t, ok := typ.Underlying().(*Basic)
return ok && t.kind == UnsafePointer
}
func isPointer(typ Type) bool {
_, ok := typ.Underlying().(*Pointer)
return ok
}
func isBytesOrRunes(typ Type) bool {
if s, ok := typ.(*Slice); ok {
t, ok := s.elem.Underlying().(*Basic)
return ok && (t.kind == Byte || t.kind == Rune)
}
return false
}