Unexpected formatting of bool-convertible custom type

[benoitryder]

If a custom type defines both operator bool and operator<<, it is formatted as bool instead of using the custom operator<<.
According to git bisect, behavior changed with ccc8f5d to fix #1316.

This seems to be the same issue as #247. (I don't know if the expected behavior changed since then.)

Example code (Godbolt):

#define FMT_HEADER_ONLY
#include <fmt/ostream.h>

struct S {
    operator bool() const { return true; }
};

std::ostream& operator<<(std::ostream& os, S const&) { return os << "s-value"; }

int main() {
    fmt::print("s: {}\n", S{});
}

Prints s: true instead of s: s-value.

[vitaut]

I don't see a way to make this work without breaking a much more important fix of #1316. Since having an implicit conversion to bool is a bad idea in general, I recommend making it explicit. Alternatively you can provide a custom formatter<S>: https://fmt.dev/latest/api.html#formatting-user-defined-types