solutions for determining if an expression meets the standard's definition of constant-expression

is_constexpr.cpp

#include <utility>
#include <string>
#include <string_view>
#include <type_traits>

using namespace std::literals;

#define version 17

// C++11 solution
#if version == 11
#define is_constexpr(...) \
    __builtin_constant_p(__VA_ARGS__)

// C++14 solution
#elif version == 14
namespace detail
{
	static auto constexpr is_constexpr_impl = [](...) {};
}

#define is_constexpr(...) \
    noexcept(detail::is_constexpr_impl(__VA_ARGS__))

// C++17 solution
#elif version == 17
#define is_constexpr(...) \
    [](auto&&... args) \
    { return std::is_invocable_v<decltype(args)&&...>; } \
    ( \
        [](auto f) -> std::bool_constant<f(__VA_ARGS__)> {}, \
        [](...) { return true; } \
    )

// C++20 solution
#elif version == 20
#define is_constexpr(...) \
    std::is_invocable_v \
    < \
        decltype([](auto f) -> std::bool_constant<f(__VA_ARGS__)> {}), \
        decltype([](...) { return true; }) \
    >

#endif

// main
int main()
{
    // empty check
    static_assert(is_constexpr());

    // literal values
    static_assert(is_constexpr("hello"));
    static_assert(is_constexpr("hello"sv));
    static_assert(!is_constexpr("hello"s));

    // constexpr values
    auto constexpr a = "hello";
    auto constexpr b = "hello"sv;    
    
    static_assert(is_constexpr(a));
    static_assert(is_constexpr(b));

    // non-constexpr values
    auto c = "hello";
    auto d = "hello"s;
    auto e = "hello"sv;
    
    static_assert(!is_constexpr(c));
    static_assert(!is_constexpr(d));
    static_assert(!is_constexpr(e));

    // non-capture lambda as literal (NOTE: relies on p0315)
    #if 0
    static_assert(is_constexpr([]{}));
    static_assert(is_constexpr([]{ return "hello"; }));
    static_assert(is_constexpr([]{ return "hello"sv; }));
    static_assert(is_constexpr([]{ return "hello"s; }));

    static_assert(is_constexpr([]{ return "hello"; }()));
    static_assert(is_constexpr([]{ return "hello"sv; }()));
    static_assert(is_constexpr([]{ return "hello"s; }()));
    #endif

    // capture lambda as literal (NOTE: relies on p0315)
    #if 0
    static_assert(is_constexpr([&]{}));
    static_assert(is_constexpr([&]{ return "hello"; }));
    static_assert(is_constexpr([&]{ return "hello"sv; }));
    static_assert(is_constexpr([&]{ return "hello"s; }));

    static_assert(is_constexpr([&]{ return "hello"; }()));
    static_assert(is_constexpr([&]{ return "hello"sv; }()));
    static_assert(is_constexpr([&]{ return "hello"s; }()));
    #endif

    // constexpr non-capture lambda
    auto constexpr lam1 = []{};
    auto constexpr lam2 = []{ return "hello"; };
    auto constexpr lam3 = []{ return "hello"sv; };
    auto constexpr lam4 = []{ return "hello"s; };

    static_assert(is_constexpr(lam1));
    static_assert(is_constexpr(lam2));
    static_assert(is_constexpr(lam3));
    static_assert(is_constexpr(lam4));

    static_assert(is_constexpr(lam2()));
    static_assert(is_constexpr(lam3()));
    static_assert(!is_constexpr(lam4()));

    // non-constexpr non-capture lambda
    auto lam5 = []{};
    auto lam6 = []{ return "hello"; };
    auto lam7 = []{ return "hello"sv; };
    auto lam8 = []{ return "hello"s; };

    static_assert(is_constexpr(lam5));
    static_assert(is_constexpr(lam6));
    static_assert(is_constexpr(lam7));
    static_assert(is_constexpr(lam8));

    static_assert(is_constexpr(lam6()));
    static_assert(is_constexpr(lam7()));
    static_assert(!is_constexpr(lam8()));

    // constexpr capture lambda
    auto constexpr cap1 = [&]{};
    auto constexpr cap2 = [&]{ return a; };

    static_assert(is_constexpr(cap1));
    static_assert(is_constexpr(cap2));

    static_assert(is_constexpr(cap2()));

    // non-constexpr capture lambda
    auto cap4 = [&]{};
    auto cap5 = [&]{ return c; };
    auto cap6 = [&]{ return d; };
    auto cap7 = [&]{ return e; };

    static_assert(is_constexpr(cap4));
    static_assert(!is_constexpr(cap5));
    static_assert(!is_constexpr(cap6));
    static_assert(!is_constexpr(cap7));

    static_assert(!is_constexpr(cap5()));
    static_assert(!is_constexpr(cap6()));
    static_assert(!is_constexpr(cap7()));
}