re PR libstdc++/68430 (std::is_constructible<T>::value == true for unconstructible type T)

PR libstdc++/68430

* testsuite/20_util/is_constructible/68430.cc: New.

From-SVN: r255785

libstdc++-v3/ChangeLog

+2017-12-18  Ville Voutilainen  <ville.voutilainen@gmail.com>
+
+	PR libstdc++/68430
+	* testsuite/20_util/is_constructible/68430.cc: New.
+
 2017-12-16  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/83239

libstdc++-v3/testsuite/20_util/is_constructible/68430.cc

+// { dg-do compile { target c++11 } }
+
+#include <type_traits>
+
+template<class T> struct Foo { Foo(T = nullptr) {} };
+static_assert(!std::is_constructible<Foo<int>>::value, "");