A custom formatter for a user-defined type might reject omitted format-specs:

```c++
struct A {};
template<>
struct std::formatter<A> {
    constexpr auto parse(std::format_parse_context const& ctx) {
        std::string_view s("narf");
        auto [i, j] = std::mismatch(ctx.begin(), ctx.end(), s.begin(), s.end());
        if (j != s.end())
            throw std::runtime_error("you didn't say the magic word!");
        return i;
    }
    auto format(A, std::format_context& ctx) const {
        return ctx.out();
    }
};
int main() {
    std::ignore = std::format("{}", A());
}
```

Per [format.fmt.string]/3 this should be rejected, but MSVC instead accepts and throws at runtime.

This implementation seems most likely to not cause problems with existing code; it matches the runtime behavior of _Default_arg_formatter on custom formatters (basic_format_arg<_Context>::handle), so it should accept any format() call that would succeed at runtime.

Co-authored-by: A. Jiang <[email protected]>