Invalid copy/move constructible/assignable detection

[deadem]

Code:

#include <https://raw.githubusercontent.com/martinmoene/optional-lite/master/include/nonstd/optional.hpp>
#include <iostream>
#include <string>

struct A
{
    A() = default;
    A(const A &) = delete;
    A& operator=(const A &) = delete;
    A(A &&) = delete;
    A& operator=(A &&) = delete;
};

int main()
{
    std::cout << "Copy constructible: "
        << std::is_copy_constructible<A>::value << " "
        << std::is_copy_constructible<nonstd::optional<A>>::value
        << std::endl;

    std::cout << "Move constructible: "
        << std::is_move_constructible<A>::value << " "
        << std::is_move_constructible<nonstd::optional<A>>::value
        << std::endl;

    std::cout << "Copy assignable: "
        << std::is_copy_assignable<A>::value << " "
        << std::is_copy_assignable<nonstd::optional<A>>::value
        << std::endl;

    std::cout << "Move assignable: "
        << std::is_move_assignable<A>::value << " "
        << std::is_move_assignable<nonstd::optional<A>>::value
        << std::endl;
}

Output:

Copy constructible: 0 1
Move constructible: 0 1
Copy assignable: 0 1
Move assignable: 0 1

Expected output:

Copy constructible: 0 0
Move constructible: 0 0
Copy assignable: 0 0
Move assignable: 0 0

Live demo: https://godbolt.org/z/a7fhv3

[martinmoene]

Added another demo at godbolt.

Relevant talks:

• How to Write Well-Behaved Value Wrappers - Simon Brand [C++ on Sea 2019]
• C++ Function Templates: How Do They Really Work? - Walter E. Brown [C++ on Sea 2019]

Output:

B / std::optional<B> - C++ std
Copy constructible: 00
Move constructible: 00
Copy assignable   : 00
Move assignable   : 00
Trivially destructable: 11

B / boost::optional<B> - Boost
Copy constructible: 01
Move constructible: 01
Copy assignable   : 01
Move assignable   : 01
Trivially destructable: 10

B / std::experimental::optional<B> - Andrzej Krzemieński
Copy constructible: 01
Move constructible: 01
Copy assignable   : 01
Move assignable   : 01
Trivially destructable: 11

B / tl::optional<B> - Sy Brand
Copy constructible: 00
Move constructible: 00
Copy assignable   : 00
Move assignable   : 00
Trivially destructable: 11

B / nonstd::optional<B> - this library
Copy constructible: 01
Move constructible: 01
Copy assignable   : 01
Move assignable   : 01
Trivially destructable: 10