Broken interaction between std::variant and ADL format_as

[Mishura4]

When formatting a variant, fmt 12.0.0 chokes if any of the alternatives relies on format_as to be formattable.
The documentation says:

A std::variant is only formattable if every variant alternative is formattable, and requires the __cpp_lib_variant library feature.

Which does not seem to align with the behavior.

Godbolt link: https://godbolt.org/z/vrjEPsbvr

Code:

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

#include <iostream>

struct foo {
    float bar;

    friend float format_as(const foo& f) { return f.bar; }
};

int main() {
    foo simple_foo{};
    std::variant<foo> variant_foo{};
    std::variant<float> variant_float{};

    std::cout << fmt::format("{}", simple_foo); // ok
    std::cout << fmt::format("{}", variant_float); // ok
    std::cout << fmt::format("{}", variant_foo); // broken
}

Error log (from Godbolt): https://pastebin.com/LXiBz4mX

[Mishura4]

The problem seems specific to fundamental types.

[vitaut]

Fixed in #4575.