go/types: delay union element checks

This is a clean port of CL 351969 from types2 to go/types
with a minor adjustment for error handling (provide an error
code).

For #46461.

Change-Id: I493dde12d8ccf86aa33f4913ac6e82f2eb459088
Reviewed-on: https://go-review.googlesource.com/c/go/+/351971
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/go/types/testdata/fixedbugs/issue46461.go2

@@ -4,8 +4,17 @@
 
 package p
 
+// test case 1
 type T[U interface{ M() T[U] }] int
 
 type X int
 
 func (X) M() T[X] { return 0 }
+
+// test case 2
+type A[T interface{ A[T] }] interface{}
+
+// test case 3
+type A2[U interface{ A2[U] }] interface{ M() A2[U] }
+
+type I interface{ A2[I]; M() A2[I] }

src/go/types/typeset.go

@@ -289,8 +289,8 @@ func computeInterfaceTypeSet(check *Checker, pos token.Pos, ityp *Interface) *_T
 			terms = tset.terms
 		case *TypeParam:
 			// Embedding stand-alone type parameters is not permitted.
-			// This case is handled during union parsing.
-			unreachable()
+			// Union parsing reports a (delayed) error, so we can ignore this entry.
+			continue
 		default:
 			if typ == Typ[Invalid] {
 				continue
@@ -370,8 +370,8 @@ func computeUnionTypeSet(check *Checker, pos token.Pos, utyp *Union) *_TypeSet {
 			terms = computeInterfaceTypeSet(check, pos, u).terms
 		case *TypeParam:
 			// A stand-alone type parameters is not permitted as union term.
-			// This case is handled during union parsing.
-			unreachable()
+			// Union parsing reports a (delayed) error, so we can ignore this entry.
+			continue
 		default:
 			if t.typ == Typ[Invalid] {
 				continue

src/go/types/union.go

@@ -57,7 +57,10 @@ func parseUnion(check *Checker, tlist []ast.Expr) Type {
 	for _, x := range tlist {
 		tilde, typ := parseTilde(check, x)
 		if len(tlist) == 1 && !tilde {
-			return typ // single type (optimization)
+			// Single type. Ok to return early because all relevant
+			// checks have been performed in parseTilde (no need to
+			// run through term validity check below).
+			return typ
 		}
 		if len(terms) >= maxTermCount {
 			check.errorf(x, _Todo, "cannot handle more than %d union terms (implementation limitation)", maxTermCount)
@@ -126,11 +129,16 @@ func parseTilde(check *Checker, x ast.Expr) (tilde bool, typ Type) {
 		tilde = true
 	}
 	typ = check.typ(x)
-	// embedding stand-alone type parameters is not permitted (issue #47127).
-	if _, ok := under(typ).(*TypeParam); ok {
-		check.error(x, _Todo, "cannot embed a type parameter")
-		typ = Typ[Invalid]
-	}
+	// Embedding stand-alone type parameters is not permitted (issue #47127).
+	// Do this check later because it requires computation of the underlying type (see also issue #46461).
+	// Note: If an underlying type cannot be a type parameter, the call to
+	//       under() will not be needed and then we don't need to delay this
+	//       check to later and could return Typ[Invalid] instead.
+	check.later(func() {
+		if _, ok := under(typ).(*TypeParam); ok {
+			check.error(x, _Todo, "cannot embed a type parameter")
+		}
+	})
 	return
 }