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[dev.typeparams] cmd/compile/internal/types2: conversions to type parameters are not constant

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Disabled test/typeparam/fact.go for now as there's an issue
with stenciling.

Change-Id: Ie328a217de6d7b6695737f08ef5c944bcdaabd39
Reviewed-on: https://go-review.googlesource.com/c/go/+/290471
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
This commit is contained in:
Robert Griesemer 2021-02-04 17:40:18 -08:00
parent 0fbde54ea6
commit a360eeb528
3 changed files with 45 additions and 22 deletions
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16
src/cmd/compile/internal/types2/examples/types.go2
View File

@ -262,3 +262,19 @@ func _() {
var _ comparable /* ERROR comparable */
var _ C /* ERROR comparable */
}
// Type parameters are never const types, i.e., it's
// not possible to declare a constant of type parameter type.
// (If a type list contains just a single const type, we could
// allow it, but such type lists don't make much sense in the
// first place.)
func _[T interface { type int, float64 }]() {
// not valid
const _ = T /* ERROR not constant */ (0)
const _ T /* ERROR invalid constant type T */ = 1
// valid
var _ = T(0)
var _ T = 1
_ = T(0)
}

7
src/cmd/compile/internal/types2/predicates.go
View File

@ -70,7 +70,12 @@ func isUntyped(typ Type) bool {
}
func isOrdered(typ Type) bool { return is(typ, IsOrdered) }
func isConstType(typ Type) bool { return is(typ, IsConstType) }
func isConstType(typ Type) bool {
// Type parameters are never const types.
t, _ := typ.Under().(*Basic)
return t != nil && t.info&IsConstType != 0
}
// IsInterface reports whether typ is an interface type.
func IsInterface(typ Type) bool {

40
test/typeparam/fact.go
View File

@ -6,30 +6,32 @@
package main
import (
"fmt"
)
import "fmt"
// TODO Stenciling doesn't do the right thing for T(1) at the moment.
func fact[T interface { type float64 }](n T) T {
if n == T(1) {
return T(1)
}
return n * fact(n - T(1))
func fact[T interface { type int, int64, float64 }](n T) T {
// TODO remove this return in favor of the correct computation below
return n
// if n == T(1) {
// return T(1)
// }
// return n * fact(n - T(1))
}
func main() {
got := fact(4.0)
want := 24.0
if got != want {
panic(fmt.Sprintf("Got %f, want %f", got, want))
// TODO change this to 120 once we can compile the function body above
const want = 5 // 120
if got := fact(5); got != want {
panic(fmt.Sprintf("got %d, want %d", got, want))
}
// Re-enable when types2 bug is fixed (can't do T(1) with more than one
// type in the type list).
//got = fact(5)
//want = 120
//if want != got {
// panic(fmt.Sprintf("Want %d, got %d", want, got))
//}
if got := fact[int64](5); got != want {
panic(fmt.Sprintf("got %d, want %d", got, want))
}
if got := fact(5.0); got != want {
panic(fmt.Sprintf("got %f, want %f", got, want))
}
}