C++ PATCH to add test for c++/87425

[Marek Polacek]

As a nice side effect, c++/87398 fixed this PR, so let's add the testcase.

Tested on x86_64-linux, applying to trunk.

2018-09-25  Marek Polacek  <pola...@redhat.com>

        PR c++/87425
        * g++.dg/cpp2a/constexpr-virtual12.C: New test.

diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-virtual12.C 
gcc/testsuite/g++.dg/cpp2a/constexpr-virtual12.C
index e69de29bb2d..8f28ecadc68 100644
--- gcc/testsuite/g++.dg/cpp2a/constexpr-virtual12.C
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-virtual12.C
@@ -0,0 +1,15 @@
+// PR c++/87425
+// P1064R0
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+struct A
+{
+  virtual A& operator= (int);
+};
+
+struct B
+{
+  A a;
+  B() { a = 0; }
+};