Compile time format string fails for user defined types in 6.0.0

[marack]

I'm finding that the compile time format strings via the _format UDL are failing for my user defined types in situations where they have previously worked. Here's a minimal example:

#include "fmt/format.h"

struct foo { };·

template <>
struct fmt::formatter<foo> : fmt::formatter<int>
{
  template <typename FmtCtx>
  auto format(foo const& f, FmCtx& ctx) { return ctx.out(); }
};

using namespace fmt::literals;

int main(int argc, char const* argv[])
{
  fmt::format("{}", foo{}); // okay
  "{}"_format(foo{}); // okay

  foo f;
  fmt::format("{}", f); // okay
  "{}"_format(f); // compilation error

  return 0;
}

Compilation error on the final UDL format which was passed a l-value foo indicates that fmt has not detected the custom formatter and is trying to use the fallback formatter:

.../fmt-6.0.0/include/fmt/format.h:2503:7: error: use of deleted function ‘fmt::v6::internal::fallback_formatter<T, Char, Enable>::fallback_formatter() [with T = foo&; Char = char; Enable = void]’

Most likely it is failing the has_formatter<T> trait. Since the custom formatter is found when passing a rvalue ("{}"_format(foo{})) perhaps there is a std::decay missing somewhere?

I'm using gcc 8.3.1, and fmt 6.0.0.

[marack]

I've found that changing the call to do_check_format_string<> in the udl_formatter template so that it decays the Args parameter pack seems to resolve the problem:

From:

do_check_format_string<Char, error_handler, Args...>(

To:

do_check_format_string<Char, error_handler, typename std::decay<Args>::type...>(

This change still passes the built in unit tests.

[vitaut]

Good catch, thanks! Fixed in 422e7b9.