c++: add fixed test [PR124389]

This one is also fixed by r16-7055.  The testcases from that commit
utilize the concepts satisfaction cache, whereas this one utilizes
the constexpr call cache.

	PR c++/124389
	PR c++/122494

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/static_assert18.C: New test.

gcc/testsuite/g++.dg/cpp0x/static_assert18.C

@@ -0,0 +1,28 @@
+// PR c++/124389
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct Empty {};
+
+template <typename T>
+constexpr bool test(T) noexcept { return true; }
+
+template <typename T>
+struct always_true {
+    static constexpr bool value = true;
+    static_assert(test(Empty<T>{}), "BUG");
+};
+
+template <typename T>
+struct AAA {
+    template <typename U, bool = always_true<U>::value>
+    AAA(U) {}
+};
+
+template <int (*U) (AAA<int>)>
+void test_function() {
+    auto r = U(AAA<int>(1));
+}
+
+int main() {
+    return 0;
+}