hana/test/sandbox/is_valid.cpp

/*
@copyright Louis Dionne 2014
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 <boost/hana/detail/static_assert.hpp>
#include <boost/hana/integral.hpp>
using namespace boost::hana;


template <typename F, typename ...X>
constexpr auto is_valid_impl(int, F f, X ...x) -> decltype(f(x...), true_)
{ return true_; }

template <typename F, typename ...X>
constexpr auto is_valid_impl(long, F, X...)
{ return false_; }


auto is_valid = [](auto f) {
    return [=](auto ...x) { return is_valid_impl(int{0}, f, x...); };
};


constexpr struct { } invalid{};

int main() {
    auto plus = [](auto x, auto y) -> decltype(x + y) { };
    BOOST_HANA_STATIC_ASSERT(is_valid(plus)(1, 2));
    BOOST_HANA_STATIC_ASSERT(!is_valid(plus)(1, invalid));
    BOOST_HANA_STATIC_ASSERT(!is_valid(plus)(invalid, 1));

    BOOST_HANA_STATIC_ASSERT(is_valid([]() -> void {})());
    BOOST_HANA_STATIC_ASSERT(!is_valid([]() -> void {})(1));
}