266.md

Info

• Did you know wrapping an unqualified function name in parentheses suppresses argument-dependent lookup?

  • http://eel.is/c++draft/basic.lookup#basic.lookup.argdep-1

Example

namespace adl {
    struct foo {};
    void bar(foo) {}
}

int main() {
    adl::foo foo;
    bar(foo);   // OK,    ADL
    (bar)(foo); // error: no ADL
}

https://godbolt.org/z/ba7Pvqa75

Puzzle

• Can you implement function bar which returns { 0: if it's not ADL call, 42: if it's a qualified call; ambigious error: if it's an ADL call }?

#include <cassert>

//TODO

int main() {
    static_assert([](auto foo) { return requires { adl::bar(foo); }; }(adl::foo{}));
    assert(42 == adl::bar(adl::foo{}));

    static_assert([](auto foo) { return requires { ::bar(foo); }; }(adl::foo{}));
    assert(0 == ::bar(adl::foo{}));

    static_assert([](auto foo) { return requires { (bar)(foo); }; }(adl::foo{}));
    assert(0 == (bar)(adl::foo{}));

    static_assert(not [](auto foo) { return requires { bar(foo); }; }(adl::foo{}));
}

https://godbolt.org/z/cxhxW9n8Y

Solutions

namespace adl {
    struct foo {};
    constexpr auto bar(foo) -> int { return 42; }
}

constexpr auto bar(adl::foo) -> int { return 0; }

https://godbolt.org/z/MPTfK7sKq

namespace adl {

struct foo {};
[[nodiscard]] constexpr auto bar(const foo) noexcept -> int { return 42; }

}  // namespace adl

[[nodiscard]] constexpr auto bar(const adl::foo) noexcept -> int { return 0; }

https://godbolt.org/z/73YbYorj3