go/types: a type parameter acts like a named type; fix predicate and test

Change-Id: Ibe9f074d516200b9b12c26a270e62defc024eeac
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go2-dev/+/769347
Reviewed-by: Robert Griesemer <gri@google.com>

src/go/types/predicates.go

@@ -15,7 +15,7 @@ import (
 // isNamed may be called with types that are not fully set up.
 func isNamed(typ Type) bool {
 	switch typ.(type) {
-	case *Basic, *Named, *instance:
+	case *Basic, *Named, *TypeParam, *instance:
 		return true
 	}
 	return false

src/go/types/testdata/issues.go2

@@ -181,13 +181,13 @@ func at(type T interface{ type []E }, E interface{})(x T, i int) E {
 }
 
 // A generic type inside a function acts like a named type. Its underlying
-// type is defined by the type list in the tybe bound, if any.
+// type is itself, its "operational type" is defined by the type list in
+// the tybe bound, if any.
 func _(type T interface{type int})(x T) {
 	type myint int
-	var _ int = x
+	var _ int = int(x)
 	var _ T = 42
-	var _ T = int(42)
-	var _ T = myint(42)
+	var _ T = T(myint(42))
 }
 
 // Indexing a generic type with an array type bound checks length.