go/types, types2: add "dynamic" flag to comparable predicate

A type implements a comparable interface only if the type
is statically known to be comparable. Specifically, a type
cannot contain (component) interfaces that are not statically
known to be comparable.

This CL adds a flag "dynamic" to the comparable predicate to
control whether interfaces are always (dynamically) comparable.
Set the flag to true when testing for (traditional) Go comparability;
set the flag to false when testing whether a type implements the
comparable interface.

Fixes #51257.

Change-Id: If22bc047ee59337deb2e7844b8f488d67e5c5530
Reviewed-on: https://go-review.googlesource.com/c/go/+/387055
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 6 insertions, 5 deletions

diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
index 14e99bf426..0360f27ee6 100644
--- a/src/go/types/predicates.go
+++ b/src/go/types/predicates.go
@@ -104,11 +104,12 @@ func isGeneric(t Type) bool {
 
 // Comparable reports whether values of type T are comparable.
 func Comparable(T Type) bool {
-	return comparable(T, nil, nil)
+	return comparable(T, true, nil, nil)
 }
 
+// If dynamic is set, non-type parameter interfaces are always comparable.
 // If reportf != nil, it may be used to report why T is not comparable.
-func comparable(T Type, seen map[Type]bool, reportf func(string, ...interface{})) bool {
+func comparable(T Type, dynamic bool, seen map[Type]bool, reportf func(string, ...interface{})) bool {
 	if seen[T] {
 		return true
 	}
@@ -126,7 +127,7 @@ func comparable(T Type, seen map[Type]bool, reportf func(string, ...interface{})
 		return true
 	case *Struct:
 		for _, f := range t.fields {
-			if !comparable(f.typ, seen, nil) {
+			if !comparable(f.typ, dynamic, seen, nil) {
 				if reportf != nil {
 					reportf("struct containing %s cannot be compared", f.typ)
 				}
@@ -135,7 +136,7 @@ func comparable(T Type, seen map[Type]bool, reportf func(string, ...interface{})
 		}
 		return true
 	case *Array:
-		if !comparable(t.elem, seen, nil) {
+		if !comparable(t.elem, dynamic, seen, nil) {
 			if reportf != nil {
 				reportf("%s cannot be compared", t)
 			}
@@ -143,7 +144,7 @@ func comparable(T Type, seen map[Type]bool, reportf func(string, ...interface{})
 		}
 		return true
 	case *Interface:
-		return !isTypeParam(T) || t.typeSet().IsComparable(seen)
+		return dynamic && !isTypeParam(T) || t.typeSet().IsComparable(seen)
 	}
 	return false
 }