diff --git a/src/go/types/testdata/typeinst2.go2 b/src/go/types/testdata/typeinst2.go2
index 6ff9b6510a..204a18cdc4 100644
--- a/src/go/types/testdata/typeinst2.go2
+++ b/src/go/types/testdata/typeinst2.go2
@@ -172,6 +172,18 @@ type _ interface {
 	type struct{f int}, struct{g int}, struct /* ERROR duplicate type */ {f int}
 }
 
+// Interface type lists can contain any type, incl. *Named types.
+// Verify that we use the underlying type to compute the operational type.
+type MyInt int
+func add1(type T interface{type MyInt})(x T) T {
+	return x + 1
+}
+
+type MyString string
+func double(type T interface{type MyInt, MyString})(x T) T {
+	return x + x
+}
+
 // Embedding of interfaces with type lists leads to interfaces
 // with type lists that are the intersection of the embedded
 // type lists.
diff --git a/src/go/types/type.go b/src/go/types/type.go
index fd1777b327..370a296c7f 100644
--- a/src/go/types/type.go
+++ b/src/go/types/type.go
@@ -828,7 +828,8 @@ func optype(typ Type) Type {
 		// (type T interface { type T }).
 		// See also issue #39680.
 		if u := t.Bound().allTypes; u != nil && u != typ {
-			return u
+			// u != typ and u is a type parameter => u.Under() != typ, so this is ok
+			return u.Under()
 		}
 		return theTop
 	}
@@ -979,7 +980,7 @@ func (t *Struct) Under() Type    { return t }
 func (t *Pointer) Under() Type   { return t }
 func (t *Tuple) Under() Type     { return t }
 func (t *Signature) Under() Type { return t }
-func (t *Sum) Under() Type       { return t }
+func (t *Sum) Under() Type       { return t } // TODO(gri) is this correct?
 func (t *Interface) Under() Type { return t }
 func (t *Map) Under() Type       { return t }
 func (t *Chan) Under() Type      { return t }