v8 regression when compiling with FMT_USE_USER_DEFINED_LITERALS=0

[mbunkus]

The following small test program can be compiled just fine with e.g. 7.1.3, but it fails with 8.0.0:

#define FMT_USE_USER_DEFINED_LITERALS 0

#include <fmt/format.h>

int main() {
  return fmt::format("hello {0}", "world").size();
}

Error message with 8.0.0 & gcc 11:

/home/mosu/prog/video/mkvtoolnix/lib/fmt/include/fmt/format.h:2752:64: error: no template named 'udl_arg' in namespace 'fmt::v7::detail'
constexpr auto operator"" _a(const char* s, size_t) -> detail::udl_arg<char> {
                                                       ~~~~~~~~^
/home/mosu/prog/video/mkvtoolnix/lib/fmt/include/fmt/format.h:2753:11: error: cannot initialize return object of type 'int' with an lvalue of type 'const char *'
  return {s};
          ^
/home/mosu/prog/video/mkvtoolnix/lib/fmt/include/fmt/format.h:2768:16: error: no template named 'udl_formatter' in namespace 'fmt::v7::detail'
    -> detail::udl_formatter<char> {
       ~~~~~~~~^
/home/mosu/prog/video/mkvtoolnix/lib/fmt/include/fmt/format.h:2769:11: warning: too many braces around scalar initializer [-Wmany-braces-around-scalar-init]
  return {{s, n}};
          ^~~~~~
/home/mosu/prog/video/mkvtoolnix/lib/fmt/include/fmt/format.h:2769:12: error: cannot initialize return object of type 'int' with an lvalue of type 'const char *'
  return {{s, n}};
           ^

I've built without user-defined literals for years now because a) my program doesn't use them and b) there are a lot of warnings with -Wpedantic and user-defined literals activated (and I'm somewhat anal about warnings).

[alexezeder]

This probably will be fixed by #2376.

[DanielaE]

May be some additional tests for these specific compilation environments are in order.

[vitaut]

Fixed in #2376, thanks for reporting.

[mbunkus]

Thanks for fixing this one so quickly in particular and all the work you've all done of this fantastic library in particular. Over the years my MKVToolNix has used the printf family, the C++ I/O streams and Boost's format before finally discovering and convert to fmt. It is so well designed, such a joy to work with, so fast and type-safe on top of it that I pretty much recommend it to everyone doing C++ things. Again, thank you all so much!

[vitaut]

Thanks @zx2c4 for the actual fix =).