Okay, perhaps not the perfect example. But the problem is that the behavior of std::variant changed between C++17 and C++20 and there's no easy way to tell at compile time which behavior std::variant with exhibit. It's the principle of the thing.

Your same "why not just leave the workaround code" argument could apply for many other language and standard library fixes which DO have a feature test macro (or an increased version of the macro).

To be clear, I'm not asking for programming help (I'd go to Stackoverflow for that). I'm raising the flag that there's perhaps an omission in the C++20 spec.

On Sun, Dec 6, 2020 at 6:34 PM Barry Revzin <barry.revzin@gmail.com> wrote:


On Sun, Dec 6, 2020 at 4:29 PM Emile Cormier <emile.cormier.jr@gmail.com> wrote:
I do, Barry. Something like this (not tested):

struct parse_event
{
    template <typename T>
    parse_event(T&& value) : token(std::forward<T>(value)) {}
   
#ifndef VARIANT_SAFELY_TAKES_CHAR_POINTER
    // Workaround
    parse_event(const char* s) : token(std::string(s)) {}
#endif
   
    std::variant<bool, int, std::string> token;
};

The idea is to disable the workaround code when the C++ library implementation has the P0608/P1957 fixes.

You could just write:

struct parse_event
{
    template <typename T>
    parse_event(T&& value) : token(std::forward<T>(value)) {}
   
    // Workaround
    parse_event(const char* s) : token(std::string(s)) {}
   
    std::variant<bool, int, std::string> token;
};

This works regardless of whether the fix is implemented.

Barry