`static_assert` fails in `format-inl.h` when using FMT_HEADER_ONLY with FMT_USE_LOCALE=0

[teruyamato0731]

Hi,

I'm using {fmt} v12.0.0 with C++20 and have encountered a compilation issue when defining both FMT_HEADER_ONLY and FMT_USE_LOCALE=0.

The Problem

When these two macros are defined simultaneously, the compilation fails with a static_assert error in fmt/format-inl.h:69.

The error seems to be triggered by including <fmt/chrono.h>. It appears that some part of the chrono formatting functionality attempts to get a std::locale, but since FMT_USE_LOCALE=0 is active, fmt::locale is a dummy detail::locale type. This type mismatch causes the static_assert(std::is_same<Locale, locale>::value, ""); to fail.

Compilation Error Log:

In file included from /path/to/project/build/_deps/fmt-src/include/fmt/format.h:4375,
                 from /path/to/project/build/_deps/fmt-src/include/fmt/base.h:2992,
                 from /path/to/project/Core/App/app.cpp:3:
/path/to/project/build/_deps/fmt-src/include/fmt/format-inl.h: In instantiation of 'Locale fmt::v12::locale_ref::get() const [with Locale = std::locale]':
/path/to/project/build/_deps/fmt-src/include/fmt/chrono.h:1598:65:   required from here
/path/to/project/build/_deps/fmt-src/include/fmt/format-inl.h:69:47: error: static assertion failed
   69 |   static_assert(std::is_same<Locale, locale>::value, "");
      |                                               ^~~~~
/path/to/project/build/_deps/fmt-src/include/fmt/format-inl.h:69:47: note: 'std::integral_constant<bool, false>::value' evaluates to false
/path/to/project/build/_deps/fmt-src/include/fmt/format-inl.h:73:10: error: could not convert 'fmt::v12::detail::locale()' from 'fmt::v12::detail::locale' to 'std::locale'
   73 |   return locale();
      |          ^~~~~~~~
      |          |
      |          fmt::v12::detail::locale
make[2]: *** [CMakeFiles/nucleo-g474re.dir/build.make:229: CMakeFiles/nucleo-g474re.dir/Core/App/app.cpp.obj] Error 1
make[1]: *** [CMakeFiles/Makefile2:119: CMakeFiles/nucleo-g474re-dokata.dir/all] Error 2
make: *** [Makefile:91: all] Error 2

---

Related ODR Violation Warning

As a related issue, if I do not define FMT_HEADER_ONLY but still define FMT_USE_LOCALE=0 in my application code (while the {fmt} library itself is likely compiled with default settings), I receive ODR (One Definition Rule) violation warnings.

This is because the layout of locale_ref and context changes based on FMT_USE_LOCALE, leading to inconsistencies between translation units.

Warning Log:

/path/to/project/build/_deps/fmt-src/include/fmt/base.h:912:7: warning: type 'struct locale_ref' violates the C++ One Definition Rule [-Wodr]
  912 | class locale_ref {
      |       ^
/path/to/project/build/_deps/fmt-src/include/fmt/base.h:912:7: note: a different type is defined in another translation unit
/path/to/project/build/_deps/fmt-src/include/fmt/base.h:915:15: note: the first difference of corresponding definitions is field 'locale_'
  915 |   const void* locale_;  // A type-erased pointer to std::locale.
      |               ^
/path/to/project/build/_deps/fmt-src/include/fmt/base.h:2668:7: warning: type 'struct context' violates the C++ One Definition Rule [-Wodr]
 2668 | class context {
      |       ^
/path/to/project/build/_deps/fmt-src/include/fmt/base.h:2668:7: note: a different type is defined in another translation unit
/path/to/project/build/_deps/fmt-src/include/fmt/base.h:2672:36: note: the first difference of corresponding definitions is field 'loc_'
 2672 |   FMT_NO_UNIQUE_ADDRESS locale_ref loc_;
      |                                    ^

It seems the combination of FMT_HEADER_ONLY and FMT_USE_LOCALE=0 is not fully compatible, especially when headers like <fmt/chrono.h> are used. Is this an intended limitation, or a bug?

Thank you

[vitaut]

Fixed in dcea616, thanks for reporting. As for the ODR violation warnings, you should compile library and application code with the same options.

[teruyamato0731]

Hi,

Thank you for the quick fix in dcea616! I can confirm that the static_assert failure is now resolved.

However, after updating to this commit, I'm now getting a new "unused parameter" warning at fmt/chrono.h:1596.

Here is the warning message:

build/_deps/fmt-src/include/fmt/chrono.h:1596:48: warning: unused parameter ‘loc’ [-Wunused-parameter]
 1596 |   inline get_locale(bool localized, locale_ref loc) : has_locale_(localized) {
      |                                     ~~~~~~~~~~~^~~

It seems the loc parameter is no longer used in that constructor when FMT_USE_LOCALE is disabled.

Also, regarding the ODR violation warnings, I understand. Thank you for the clarification that the library and application code must be compiled with the same options.

[vitaut]

The warning should be fixed in 5f66e07.