go/types/typeutil: implement correct hashing of aliases

Only the Alias constructors should be ignored, not any
Named ones.

(The existing map_test caught this.)

Change-Id: I8593bd6e7a6a00b9ea5921e402c61db948fad6bd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/577575
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

go/types/typeutil/map.go

@@ -261,7 +261,7 @@ func (h Hasher) hashFor(t types.Type) uint32 {
 		return uint32(t.Kind())
 
 	case *aliases.Alias:
-		return h.Hash(t.Underlying())
+		return h.Hash(aliases.Unalias(t))
 
 	case *types.Array:
 		return 9043 + 2*uint32(t.Len()) + 3*h.Hash(t.Elem())
@@ -462,7 +462,7 @@ func (h Hasher) shallowHash(t types.Type) uint32 {
 	// so there's no need to optimize anything else.
 	switch t := t.(type) {
 	case *aliases.Alias:
-		return h.shallowHash(t.Underlying())
+		return h.shallowHash(aliases.Unalias(t))
 
 	case *types.Signature:
 		var hash uint32 = 604171