[dev.typeparams] cmd/compile/internal/types2: review of predicates.go

Make predicates.go match the corresponding and reviewed go/types version.

The remaining diffs are due to the difference in the implementations
of the type conversion methods/functions:

$ diff $GOROOT/src/cmd/compile/internal/types2/predicates.go $GOROOT/src/go/types/predicates.go
7c7
< package types2
---
> package types
9a10
> 	"go/token"
32c33
< 	switch t := optype(typ.Under()).(type) {
---
> 	switch t := optype(typ).(type) {
63c64
< 	// set up. Must not call Basic()!
---
> 	// set up. Must not call asBasic()!
79c80
< 	t := typ.Basic()
---
> 	t := asBasic(typ)
85c86
< 	return typ.Interface() != nil
---
> 	return asInterface(typ) != nil
110c111
< 	if t := T.TypeParam(); t != nil && optype(t) == theTop {
---
> 	if t := asTypeParam(T); t != nil && optype(t) == theTop {
114c115
< 	switch t := optype(T.Under()).(type) {
---
> 	switch t := optype(T).(type) {
143c144
< 	switch t := optype(typ.Under()).(type) {
---
> 	switch t := optype(typ).(type) {
300,301c301,302
< 				check.completeInterface(nopos, x)
< 				check.completeInterface(nopos, y)
---
> 				check.completeInterface(token.NoPos, x)
> 				check.completeInterface(token.NoPos, y)

Change-Id: I174d8a8a22fbd8814ede25002cb2705588912329
Reviewed-on: https://go-review.googlesource.com/c/go/+/278474
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -1,4 +1,3 @@
-// UNREVIEWED
 // Copyright 2012 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.
@@ -21,7 +20,8 @@ func isNamed(typ Type) bool {
 	return false
 }
 
-// isGeneric reports whether a type is a generic, uninstantiated type (generic signatures are not included).
+// isGeneric reports whether a type is a generic, uninstantiated type (generic
+// signatures are not included).
 func isGeneric(typ Type) bool {
 	// A parameterized type is only instantiated if it doesn't have an instantiation already.
 	named, _ := typ.(*Named)
@@ -90,9 +90,16 @@ func Comparable(T Type) bool {
 	return comparable(T, nil)
 }
 
-// comparable should only be called by Comparable.
 func comparable(T Type, seen map[Type]bool) bool {
-	// If T is a type parameter not constraint by any type
+	if seen[T] {
+		return true
+	}
+	if seen == nil {
+		seen = make(map[Type]bool)
+	}
+	seen[T] = true
+
+	// If T is a type parameter not constrained by any type
 	// list (i.e., it's underlying type is the top type),
 	// T is comparable if it has the == method. Otherwise,
 	// the underlying type "wins". For instance
@@ -104,14 +111,6 @@ func comparable(T Type, seen map[Type]bool) bool {
 		return t.Bound().IsComparable()
 	}
 
-	if seen[T] {
-		return true
-	}
-	if seen == nil {
-		seen = make(map[Type]bool)
-	}
-	seen[T] = true
-
 	switch t := optype(T.Under()).(type) {
 	case *Basic:
 		// assume invalid types to be comparable
@@ -129,7 +128,10 @@ func comparable(T Type, seen map[Type]bool) bool {
 	case *Array:
 		return comparable(t.elem, seen)
 	case *Sum:
-		return t.is(Comparable)
+		pred := func(t Type) bool {
+			return comparable(t, seen)
+		}
+		return t.is(pred)
 	case *TypeParam:
 		return t.Bound().IsComparable()
 	}