adding can_invoke_constexpr example for member function pointer

example/can_invoke_constexpr/member_function_pointer.cpp

@@ -0,0 +1,37 @@
+
+/*!
+Copyright (c) 2016 Barrett Adair
+
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+*/
+
+#include <type_traits>
+#include <callable_traits/callable_traits.hpp>
+
+// NOTE: Due to non-compliance in MSVC, can_invoke_constexpr
+// always returns std::false_type on that compiler, which
+// causes a static assert below to fail.
+
+namespace ct = callable_traits;
+
+struct foo {
+    constexpr int bar(int) const {
+        return 1;
+    }
+};
+
+using pmf_constant = std::integral_constant<decltype(&foo::bar), &foo::bar>;
+
+// can_invoke_constexpr returns true here because foo::bar
+// is constexpr, and the arguments are valid to INVOKE
+static_assert(ct::can_invoke_constexpr(pmf_constant{}, foo{}, 0), "");
+
+// can_invoke_constexpr returns false here because even though
+// foo::bar is constexpr, the arguments do not obey INVOKE rules
+static_assert(!ct::can_invoke_constexpr(pmf_constant{}, foo{}), "");
+
+// Don't forget that all arguments to can_invoke_constexpr
+// must be trivially default constructible
+
+int main() { return 0; }