cmd/compile, go/*: move method type parameter checks from parsers to type checkers

The parsers (cmd/compile/internal/syntax and go/parser) always accepted
type parameters on methods for parser robustness but reported an error.

With this change, the parsers accept the type parameters on methods,
and then the type checkers (cmd/compile/internal/types2 and go/types)
complain about them.

Add test case for method type parameters when running the parsers only.
Adjust some existing test cases as needed.

This change is a necessary first step towards implementing generic
methods but does not enable them.

For #77273.

Change-Id: I291fcf0aef0c917c74b32131c88b9e4ed71c5060
Reviewed-on: https://go-review.googlesource.com/c/go/+/738441
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/syntax/testdata/smoketest.go b/src/cmd/compile/internal/syntax/testdata/smoketest.go
index 6b3593ac7a..e7c9f7283f 100644
--- a/src/cmd/compile/internal/syntax/testdata/smoketest.go
+++ b/src/cmd/compile/internal/syntax/testdata/smoketest.go
@@ -71,3 +71,33 @@ type _ interface {
 	T[P]
 	T[P1, P2]
 }
+
+// generic method
+type List[E any] []E
+
+func (l List[E]) Map[F any](m func(E) F) (r List[F]) {
+	for _, x := range l {
+		r = append(r, m(x))
+	}
+	return
+}
+
+func _() {
+	l := List[string]{"foo", "foobar", "42"}
+	r := l.Map(func(s string) int { return len(s)})
+	_ = r
+}
+
+func _[E, F any](l List[E]) List[F] {
+	var f func(List[E], func(E) F) List[F] = List[E].Map  // method expression & type inference
+	return f(l, func(E) F { var f F; return f })
+}
+
+// disallowed type parameters
+
+type _ func /* ERROR function type must have no type parameters */ [P any](P)
+type _ interface {
+	m /* ERROR interface method must have no type parameters */ [P any](P)
+}
+
+var _ = func /* ERROR function type must have no type parameters */ [P any](P) {}