// zeroConst returns a new "zero" constant of the specified type,
// which must not be an array or struct type: the zero values of
// aggregates are well-defined but cannot be represented by Const.
//
func zeroConst(t types.Type) *Const {
switch t := t.(type) {
case *types.Basic:
switch {
case t.Info()&types.IsBoolean != 0:
return NewConst(exact.MakeBool(false), t)
case t.Info()&types.IsNumeric != 0:
return NewConst(exact.MakeInt64(0), t)
case t.Info()&types.IsString != 0:
return NewConst(exact.MakeString(""), t)
case t.Kind() == types.UnsafePointer:
fallthrough
case t.Kind() == types.UntypedNil:
return nilConst(t)
default:
panic(fmt.Sprint("zeroConst for unexpected type:", t))
}
case *types.Pointer, *types.Slice, *types.Interface, *types.Chan, *types.Map, *types.Signature:
return nilConst(t)
case *types.Named:
return NewConst(zeroConst(t.Underlying()).Value, t)
case *types.Array, *types.Struct, *types.Tuple:
panic(fmt.Sprint("zeroConst applied to aggregate:", t))
}
panic(fmt.Sprint("zeroConst: unexpected ", t))
}
// 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 x.isInteger() && isString(t):
codepoint := int64(-1)
if i, ok := exact.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 = exact.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
}
// stringConst returns a 'string' constant that evaluates to s.
func stringConst(s string) *Const {
return NewConst(exact.MakeString(s), tString)
}