LWG 3636

[ecatmur]

Per https://cplusplus.github.io/LWG/issue3636, formatters should have format() const. Failure to enforce this impedes compatibility with standard formatting i.e. once we've written formatters for fmt, we need to go and add const on format() to make them compatible with std::format.

Please consider enforcing that formatter<T>::format() specialization member functions can be called const. Perhaps this could be done with deprecation initially.

[ecatmur]

#include <fmt/format.h>
#include <format>

struct A {};
template<>
struct fmt::formatter<A> {
    auto format(A, auto& ctx) /* const */ -> decltype(ctx.out());
    constexpr auto parse(auto& ctx) { return ctx.begin(); }
};
template<>
struct std::formatter<A> : fmt::formatter<A> {};

int main() {
    (void) fmt::format("{}", A()); // 1
    (void) std::format("{}", A()); // 2
}

compiles (1) but fails to compile (2). Uncommenting the /* const */ makes both compile. https://godbolt.org/z/jWPGs49Ka

[infdahai]

maybe just add the const flag to the template.

[vitaut]

Done in d707292.