C++ PATCH to add test for c++/79520

[Marek Polacek]

Fixed by r255788.

Tested on x86_64-linux, applying to trunk.

2019-08-08  Marek Polacek  <pola...@redhat.com>

        PR c++/79520
        * g++.dg/cpp1y/constexpr-79520.C: New test.

--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-79520.C
@@ -0,0 +1,11 @@
+// PR c++/79520
+// { dg-do compile { target c++14 } }
+
+constexpr int f(int const& x) { return x; }
+
+constexpr struct S {
+    int x = 0;
+    constexpr S() {(void)f(x); x = 1;}
+} s;
+
+static_assert(f(s.x) == 1, "");