cmd/compile/internal/types2: produce empty type set for invalid ~T

If ~T is not permitted because the underlying type of T is not the
same as T, there is no type that satisfies ~T. Besides reporting an
error, also ensure that the corresponding type set is empty.

For #49739.

Change-Id: I127f75f170902e7989f7fe7b352dabda9f72e2a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/366278
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/testdata/fixedbugs/issue49739.go2

@@ -0,0 +1,23 @@
+// Copyright 2021 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.
+
+// Verify that we get an empty type set (not just an error)
+// when using an invalid ~A.
+
+package p
+
+type A int
+type C interface {
+	~ /* ERROR invalid use of ~ */ A
+}
+
+func f[_ C]()              {}
+func g[_ interface{ C }]() {}
+func h[_ C | int]()        {}
+
+func _() {
+	_ = f[int /* ERROR cannot implement C \(empty type set\) */]
+	_ = g[int /* ERROR cannot implement interface{C} \(empty type set\) */]
+	_ = h[int]
+}

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

@@ -367,14 +367,18 @@ func computeUnionTypeSet(check *Checker, pos syntax.Pos, utyp *Union) *_TypeSet
 	var allTerms termlist
 	for _, t := range utyp.terms {
 		var terms termlist
-		switch u := under(t.typ).(type) {
-		case *Interface:
+		u := under(t.typ)
+		if ui, _ := u.(*Interface); ui != nil {
 			// For now we don't permit type parameters as constraints.
 			assert(!isTypeParam(t.typ))
-			terms = computeInterfaceTypeSet(check, pos, u).terms
-		default:
-			if t.typ == Typ[Invalid] {
+			terms = computeInterfaceTypeSet(check, pos, ui).terms
+		} else if t.typ == Typ[Invalid] {
 			continue
+		} else {
+			if t.tilde && !Identical(t.typ, u) {
+				// There is no underlying type which is t.typ.
+				// The corresponding type set is empty.
+				t = nil // ∅ term
 			}
 			terms = termlist{(*term)(t)}
 		}