cmd/compile/internal/types2: fix TypeName.IsAlias for type parameter names

This is a port of CL 359656 from go/types to types2.

For #49213.

Change-Id: Ib98f9a344c1397af92e061cafd519ea374fd60bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/360294
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -315,6 +315,8 @@ func (obj *TypeName) IsAlias() bool {
 		return obj.pkg != nil || t.name != obj.name || t == universeByte || t == universeRune
 	case *Named:
 		return obj != t.obj
+	case *TypeParam:
+		return obj != t.obj
 	default:
 		return true
 	}

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

@@ -33,6 +33,8 @@ func TestIsAlias(t *testing.T) {
 	pkg := NewPackage("p", "p")
 	t1 := NewTypeName(nopos, pkg, "t1", nil)
 	n1 := NewNamed(t1, new(Struct), nil)
+	t5 := NewTypeName(nopos, pkg, "t5", nil)
+	NewTypeParam(t5, nil)
 	for _, test := range []struct {
 		name  *TypeName
 		alias bool
@@ -46,6 +48,7 @@ func TestIsAlias(t *testing.T) {
 		{NewTypeName(nopos, nil, "int32", Typ[Int32]), false},  // type name refers to basic type with same name
 		{NewTypeName(nopos, pkg, "int32", Typ[Int32]), true},   // type name is declared in user-defined package (outside Universe)
 		{NewTypeName(nopos, nil, "rune", Typ[Rune]), true},     // type name refers to basic type rune which is an alias already
+		{t5, false}, // type name refers to type parameter and vice versa
 	} {
 		check(test.name, test.alias)
 	}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue45985.go2

@@ -5,7 +5,7 @@
 package issue45985
 
 // TODO(gri): this error should be on app[int] below.
-func app[S /* ERROR "type S = S does not match" */ interface{ ~[]T }, T any](s S, e T) S {
+func app[S /* ERROR "type S S does not match" */ interface{ ~[]T }, T any](s S, e T) S {
     return append(s, e)
 }