[dev.typeparams] cmd/compile/internal/types2: conversions to type par…

…ameters are not constant

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>

src/cmd/compile/internal/types2/examples/types.go2

@@ -261,4 +261,20 @@ func _(_ comparable /* ERROR comparable */ , _ C /* ERROR comparable */ )
 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)
+}

src/cmd/compile/internal/types2/predicates.go

@@ -69,8 +69,13 @@ func isUntyped(typ Type) bool {
 	return !isTyped(typ)
 }
 
-func isOrdered(typ Type) bool   { return is(typ, IsOrdered) }
-func isConstType(typ Type) bool { return is(typ, IsConstType) }
+func isOrdered(typ Type) bool { return is(typ, IsOrdered) }
+
+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 {

test/typeparam/fact.go

@@ -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))
+	}
 }