go/types, types2: fix implements and identical predicates

- Use the correct predicate in Checker.implements: for interfaces
  we cannot use the API Comparable because it always returns true
  for all non-type parameter interface types: Comparable simply
  answers if == and != is permitted, and it's always been permitted
  for interfaces. Instead we must use Interface.IsComparable which
  looks at the type set of an interface.

- When comparing interfaces for identity, we must also consider the
  whether the type sets have the comparable bit set.

With this change, `any` doesn't implement `comparable` anymore. This
only matters for generic functions and types, and the API functions.
It does mean that for now (until we allow type-constrained interfaces
for general non-constraint use, at some point in the future) a type
parameter that needs to be comparable cannot be instantiated with an
interface anymore.

For #50646.

Change-Id: I7e7f711bdcf94461f330c90509211ec0c2cf3633
Reviewed-on: https://go-review.googlesource.com/c/go/+/381254
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/types2/predicates.go b/src/cmd/compile/internal/types2/predicates.go
index cc3c76e695..003e58db38 100644
--- a/src/cmd/compile/internal/types2/predicates.go
+++ b/src/cmd/compile/internal/types2/predicates.go
@@ -306,6 +306,9 @@ func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
 		if y, ok := y.(*Interface); ok {
 			xset := x.typeSet()
 			yset := y.typeSet()
+			if xset.comparable != yset.comparable {
+				return false
+			}
 			if !xset.terms.equal(yset.terms) {
 				return false
 			}