[dev.go2go] go/types: a type constraint must be an interface

In particular, it cannot be a type parameter
with an operational type that is an interface.

Fixes #39723.

Change-Id: Ia6b65eaecd472dd71b1c9f7631ce1872f06e5a6d
Reviewed-on: https://go-review.googlesource.com/c/go/+/239161
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/go/types/check_test.go

@@ -129,6 +129,7 @@ var tests = [][]string{
 	{"fixedbugs/issue39693.go2"},
 	{"fixedbugs/issue39680.go2"},
 	{"fixedbugs/issue39711.go2"},
+	{"fixedbugs/issue39723.go2"},
 }
 
 var fset = token.NewFileSet()

src/go/types/decl.go

@@ -615,17 +615,19 @@ func (check *Checker) collectTypeParams(list *ast.FieldList) (tparams []*TypeNam
 	}
 
 	index := 0
+	var bound Type
 	for _, f := range list.List {
 		if f.Type == nil {
 			goto next
 		}
 
 		// type bound must be an interface
-		// TODO(gri) We should try to delay the IsInterface check
-		//           as it may expand a possibly incomplete type.
-		//           Worse, it could be a type parameter that is
-		//           not yet instantiated if we accept #39723.
-		if bound := check.anyType(f.Type); IsInterface(bound) {
+		// TODO(gri) We should delay the interface check because
+		//           we may not have a complete interface yet:
+		//           type C(type T C) interface {}
+		//           (issue #39724).
+		bound = check.anyType(f.Type)
+		if _, ok := bound.Under().(*Interface); ok {
 			// If the type bound expects exactly one type argument, permit leaving
 			// it away and use the corresponding type parameter as implicit argument.
 			// This allows us to write (type p b(p), q b(q), r b(r)) as (type p, q, r b).

src/go/types/fixedbugs/issue39723.go2

@@ -0,0 +1,9 @@
+// Copyright 2020 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.
+
+package p
+
+// A constraint must be an interface; it cannot
+// be a type parameter, for instance.
+func _(type A interface{ type interface{} }, B A /* ERROR not an interface */ )()