[dev.go2go] go/types: avoid endless recursion when computing operational types

The operational type for a type parameter T constrained by
interface { type T } is the top type, not itself. The latter
can lead to infinite recursions.

Fix suggested by @tdakkota.

Fixes #39680.

Change-Id: I8a6a6c503f09294b9276965f1d9e05c5d22ec912
Reviewed-on: https://go-review.googlesource.com/c/go/+/238863
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/go/types/check_test.go

@@ -127,6 +127,7 @@ var tests = [][]string{
 	//           Should just traverse that directory.
 	{"fixedbugs/issue39664.go2"},
 	{"fixedbugs/issue39693.go2"},
+	{"fixedbugs/issue39680.go2"},
 }
 
 var fset = token.NewFileSet()

src/go/types/fixedbugs/issue39680.go2

@@ -0,0 +1,27 @@
+// 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
+
+import "fmt"
+
+// Minimal test case.
+func _(type T interface{type T})(x T) T{
+	return x
+}
+
+// Test case from issue.
+type constr(type T) interface {
+	type T
+}
+
+func Print(type T constr(T))(s []T) {
+	for _, v := range s {
+		fmt.Print(v)
+	}
+}
+
+func f() {
+	Print([]string{"Hello, ", "playground\n"})
+}

src/go/types/type.go

@@ -839,10 +839,17 @@ func (t *TypeParam) Bound() *Interface {
 // as the underlying type (as returned by Under). For
 // Type parameters, the operational type is determined
 // by the corresponding type bound's type list. The
-// result may be the bottom or top type.
+// result may be the bottom or top type, but it is never
+// the incoming type parameter.
 func optype(typ Type) Type {
 	if t := typ.TypeParam(); t != nil {
-		if u := t.Bound().allTypes; u != nil {
+		// If the optype is typ, return the top type as we have
+		// no information. It also prevents infinite recursion
+		// via the TypeParam converter methods. This can happen
+		// for a type parameter list of the form:
+		// (type T interface { type T }).
+		// See also issue #39680.
+		if u := t.Bound().allTypes; u != nil && u != typ {
 			return u
 		}
 		return theTop