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>

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

@@ -221,7 +221,7 @@ func (check *Checker) implements(V, T Type) error {
 	// If T is comparable, V must be comparable.
 	// Remember as a pending error and report only if we don't have a more specific error.
 	var pending error
-	if Ti.IsComparable() && !Comparable(V) {
+	if Ti.IsComparable() && ((Vi != nil && !Vi.IsComparable()) || (Vi == nil && !Comparable(V))) {
 		pending = errorf("%s does not implement comparable", V)
 	}
 

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

@@ -623,16 +623,15 @@ func TestIssue50646(t *testing.T) {
 		t.Errorf("comparable is not a comparable type")
 	}
 
-	// TODO(gri) should comparable be an alias, like any? (see #50791)
-	if !Implements(anyType, comparableType.Underlying().(*Interface)) {
-		t.Errorf("any does not implement comparable")
+	if Implements(anyType, comparableType.Underlying().(*Interface)) {
+		t.Errorf("any implements comparable")
 	}
 	if !Implements(comparableType, anyType.(*Interface)) {
 		t.Errorf("comparable does not implement any")
 	}
 
-	if !AssignableTo(anyType, comparableType) {
-		t.Errorf("any not assignable to comparable")
+	if AssignableTo(anyType, comparableType) {
+		t.Errorf("any assignable to comparable")
 	}
 	if !AssignableTo(comparableType, anyType) {
 		t.Errorf("comparable not assignable to any")

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
 			}

src/cmd/compile/internal/types2/testdata/check/issues.go2

@@ -9,19 +9,18 @@ package p
 import "io"
 import "context"
 
-// Interfaces are always comparable (though the comparison may panic at runtime).
 func eql[T comparable](x, y T) bool {
 	return x == y
 }
 
-func _() {
-	var x interface{}
-	var y interface{ m() }
+func _[X comparable, Y interface{comparable; m()}]() {
+	var x X
+	var y Y
 	eql(x, y /* ERROR does not match */ ) // interfaces of different types
 	eql(x, x)
 	eql(y, y)
-	eql(y, nil)
-	eql[io.Reader](nil, nil)
+	eql(y, nil /* ERROR cannot use nil as Y value in argument to eql */ )
+	eql[io /* ERROR does not implement comparable */ .Reader](nil, nil)
 }
 
 // If we have a receiver of pointer to type parameter type (below: *T)

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50646.go2

@@ -4,9 +4,6 @@
 
 package p
 
-// Because we can use == and != with values of arbitrary
-// interfaces, all interfaces implement comparable.
-
 func f1[_ comparable]()              {}
 func f2[_ interface{ comparable }]() {}
 
@@ -14,15 +11,15 @@ type T interface{ m() }
 
 func _[P comparable, Q ~int, R any]() {
 	_ = f1[int]
-	_ = f1[T]
-	_ = f1[any]
+	_ = f1[T /* ERROR T does not implement comparable */ ]
+	_ = f1[any /* ERROR any does not implement comparable */ ]
 	_ = f1[P]
 	_ = f1[Q]
 	_ = f1[R /* ERROR R does not implement comparable */]
 
 	_ = f2[int]
-	_ = f2[T]
-	_ = f2[any]
+	_ = f2[T /* ERROR T does not implement comparable */ ]
+	_ = f2[any /* ERROR any does not implement comparable */ ]
 	_ = f2[P]
 	_ = f2[Q]
 	_ = f2[R /* ERROR R does not implement comparable */]

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

@@ -387,6 +387,9 @@ func (u *unifier) nify(x, y Type, 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
 			}

src/go/types/instantiate.go

@@ -225,7 +225,7 @@ func (check *Checker) implements(V, T Type) error {
 	// If T is comparable, V must be comparable.
 	// Remember as a pending error and report only if we don't have a more specific error.
 	var pending error
-	if Ti.IsComparable() && !Comparable(V) {
+	if Ti.IsComparable() && ((Vi != nil && !Vi.IsComparable()) || (Vi == nil && !Comparable(V))) {
 		pending = errorf("%s does not implement comparable", V)
 	}
 

src/go/types/issues_test.go

@@ -650,16 +650,15 @@ func TestIssue50646(t *testing.T) {
 		t.Errorf("comparable is not a comparable type")
 	}
 
-	// TODO(gri) should comparable be an alias, like any? (see #50791)
-	if !Implements(anyType, comparableType.Underlying().(*Interface)) {
-		t.Errorf("any does not implement comparable")
+	if Implements(anyType, comparableType.Underlying().(*Interface)) {
+		t.Errorf("any implements comparable")
 	}
 	if !Implements(comparableType, anyType.(*Interface)) {
 		t.Errorf("comparable does not implement any")
 	}
 
-	if !AssignableTo(anyType, comparableType) {
-		t.Errorf("any not assignable to comparable")
+	if AssignableTo(anyType, comparableType) {
+		t.Errorf("any assignable to comparable")
 	}
 	if !AssignableTo(comparableType, anyType) {
 		t.Errorf("comparable not assignable to any")

src/go/types/predicates.go

@@ -308,6 +308,9 @@ 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
 			}

src/go/types/testdata/check/issues.go2

@@ -9,19 +9,18 @@ package p
 import "io"
 import "context"
 
-// Interfaces are always comparable (though the comparison may panic at runtime).
 func eql[T comparable](x, y T) bool {
 	return x == y
 }
 
-func _() {
-	var x interface{}
-	var y interface{ m() }
+func _[X comparable, Y interface{comparable; m()}]() {
+	var x X
+	var y Y
 	eql(x, y /* ERROR does not match */ ) // interfaces of different types
 	eql(x, x)
 	eql(y, y)
-	eql(y, nil)
-	eql[io.Reader](nil, nil)
+	eql(y, nil /* ERROR cannot use nil as Y value in argument to eql */ )
+	eql[io /* ERROR does not implement comparable */ .Reader](nil, nil)
 }
 
 // If we have a receiver of pointer to type parameter type (below: *T)

src/go/types/testdata/fixedbugs/issue50646.go2

@@ -4,9 +4,6 @@
 
 package p
 
-// Because we can use == and != with values of arbitrary
-// interfaces, all interfaces implement comparable.
-
 func f1[_ comparable]()              {}
 func f2[_ interface{ comparable }]() {}
 
@@ -14,15 +11,15 @@ type T interface{ m() }
 
 func _[P comparable, Q ~int, R any]() {
 	_ = f1[int]
-	_ = f1[T]
-	_ = f1[any]
+	_ = f1[T /* ERROR T does not implement comparable */ ]
+	_ = f1[any /* ERROR any does not implement comparable */ ]
 	_ = f1[P]
 	_ = f1[Q]
 	_ = f1[R /* ERROR R does not implement comparable */]
 
 	_ = f2[int]
-	_ = f2[T]
-	_ = f2[any]
+	_ = f2[T /* ERROR T does not implement comparable */ ]
+	_ = f2[any /* ERROR any does not implement comparable */ ]
 	_ = f2[P]
 	_ = f2[Q]
 	_ = f2[R /* ERROR R does not implement comparable */]

src/go/types/unify.go

@@ -387,6 +387,9 @@ func (u *unifier) nify(x, y Type, 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
 			}

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 {

test/typeparam/issue48276a.out

@@ -1 +1 @@
-<nil>:
+0:0

test/typeparam/issue50646.go

@@ -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) })
-}