go/types, types2: all interfaces implement comparable (add tests)

For #50646.

Change-Id: I7420545556e0df2659836364a62ce2c32ad7a8b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/380654
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -611,3 +611,30 @@ func TestIssue43124(t *testing.T) {
 		t.Errorf("type checking error for c does not disambiguate package template: %q", err)
 	}
 }
+
+func TestIssue50646(t *testing.T) {
+	anyType := Universe.Lookup("any").Type()
+	comparableType := Universe.Lookup("comparable").Type()
+
+	if !Comparable(anyType) {
+		t.Errorf("any is not a comparable type")
+	}
+	if !Comparable(comparableType) {
+		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(comparableType, anyType.(*Interface)) {
+		t.Errorf("comparable does not implement any")
+	}
+
+	if !AssignableTo(anyType, comparableType) {
+		t.Errorf("any not assignable to comparable")
+	}
+	if !AssignableTo(comparableType, anyType) {
+		t.Errorf("comparable not assignable to any")
+	}
+}

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

@@ -0,0 +1,29 @@
+// 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 p
+
+// Because we can use == and != with values of arbitrary
+// interfaces, all interfaces implement comparable.
+
+func f1[_ comparable]()              {}
+func f2[_ interface{ comparable }]() {}
+
+type T interface{ m() }
+
+func _[P comparable, Q ~int, R any]() {
+	_ = f1[int]
+	_ = f1[T]
+	_ = f1[any]
+	_ = f1[P]
+	_ = f1[Q]
+	_ = f1[R /* ERROR R does not implement comparable */]
+
+	_ = f2[int]
+	_ = f2[T]
+	_ = f2[any]
+	_ = f2[P]
+	_ = f2[Q]
+	_ = f2[R /* ERROR R does not implement comparable */]
+}

src/go/types/issues_test.go

@@ -638,3 +638,30 @@ var _ T = template /* ERROR cannot use.*text/template.* as T value */.Template{}
 	testFiles(t, nil, []string{"c.go"}, [][]byte{[]byte(csrc)}, false, imp)
 	testFiles(t, nil, []string{"t.go"}, [][]byte{[]byte(tsrc)}, false, imp)
 }
+
+func TestIssue50646(t *testing.T) {
+	anyType := Universe.Lookup("any").Type()
+	comparableType := Universe.Lookup("comparable").Type()
+
+	if !Comparable(anyType) {
+		t.Errorf("any is not a comparable type")
+	}
+	if !Comparable(comparableType) {
+		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(comparableType, anyType.(*Interface)) {
+		t.Errorf("comparable does not implement any")
+	}
+
+	if !AssignableTo(anyType, comparableType) {
+		t.Errorf("any not assignable to comparable")
+	}
+	if !AssignableTo(comparableType, anyType) {
+		t.Errorf("comparable not assignable to any")
+	}
+}

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

@@ -0,0 +1,29 @@
+// 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 p
+
+// Because we can use == and != with values of arbitrary
+// interfaces, all interfaces implement comparable.
+
+func f1[_ comparable]()              {}
+func f2[_ interface{ comparable }]() {}
+
+type T interface{ m() }
+
+func _[P comparable, Q ~int, R any]() {
+	_ = f1[int]
+	_ = f1[T]
+	_ = f1[any]
+	_ = f1[P]
+	_ = f1[Q]
+	_ = f1[R /* ERROR R does not implement comparable */]
+
+	_ = f2[int]
+	_ = f2[T]
+	_ = f2[any]
+	_ = f2[P]
+	_ = f2[Q]
+	_ = f2[R /* ERROR R does not implement comparable */]
+}

test/typeparam/issue50646.go

@@ -0,0 +1,39 @@
+// 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) })
+}