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>

3 files changed, 2 insertions, 41 deletions

diff --git a/test/typeparam/issue48276a.go b/test/typeparam/issue48276a.go
index 060ac3eb7f..25e939f536 100644
--- a/test/typeparam/issue48276a.go
+++ b/test/typeparam/issue48276a.go
@@ -9,7 +9,7 @@ package main
 import "fmt"
 
 func main() {
-	IsZero[interface{}]("")
+	IsZero[int](0)
 }
 
 func IsZero[T comparable](val T) bool {
diff --git a/test/typeparam/issue48276a.out b/test/typeparam/issue48276a.out
index 7e8a8a9a2e..8f38db999d 100644
--- a/test/typeparam/issue48276a.out
+++ b/test/typeparam/issue48276a.out
@@ -1 +1 @@
-<nil>:
+0:0
diff --git a/test/typeparam/issue50646.go b/test/typeparam/issue50646.go
deleted file mode 100644
index 44bbe2ae6f..0000000000
--- a/test/typeparam/issue50646.go
+++ /dev/null
@@ -1,39 +0,0 @@
-// run -gcflags=-G=3
-
-// Copyright 2022 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package main
-
-func eql[P comparable](x, y P) {
-	if x != y {
-		panic("not equal")
-	}
-}
-
-func expectPanic(f func()) {
-	defer func() {
-		if recover() == nil {
-			panic("function succeeded unexpectedly")
-		}
-	}()
-	f()
-}
-
-func main() {
-	eql[int](1, 1)
-	eql(1, 1)
-
-	// all interfaces implement comparable
-	var x, y any = 2, 2
-	eql[any](x, y)
-	eql(x, y)
-
-	// but we may get runtime panics
-	x, y = 1, 2 // x != y
-	expectPanic(func() { eql(x, y) })
-
-	x, y = main, main // functions are not comparable
-	expectPanic(func() { eql(x, y) })
-}