Do not #undef __STRICT_ANSI__

[jwakely]

This:

fmt/include/fmt/os.h

Line 13 in 25a41b8

# undef __STRICT_ANSI__

causes undefined behaviour. That macro is for the implementation's use, not for yours. Changing it can lead to inconsistent state in libstdc++ headers, e.g. if somebody does this:

#include <type_traits>
#include <fmt/os.h>
#include <random>

If somebody compiles this snippet with -std=c++17 then __STRICT_ANSI__ will be defined when <type_traits> is included, so libstdc++ will define things in "strict" mode (e.g. is_integral_v<__int128> is false) and then the macro gets undefined, and then <random> defines things in "non-strict" mode, so it assumes it can use __int128 as a proper integer type. But it doesn't work, because of the earlier strict definitions in <type_traits>. You get confusing and nonsensical errors from libstdc++ headers.

Do not mess with this macro.

If your code is not compatible with __STRICT_ANSI__ then just tell users they need to use -std=gnu++17 not -std=c++17 so that __STRICT_ANSI__ isn't defined in the first place.

[vitaut]

I don't think this workaround is even used anymore. Removed in e737672, thanks for reporting.

[yachoor]

Any chance to get it backported to 4.x branch or is it no longer supported?

fmt 4.1.0 stopped building after updating gcc to 14.1.0 (and my project has to also build with RVCT 4.1 and gcc 4.4.1, so I'm stuck on that branch and 15 years in the past 😭):

In file included from C:/msys64/mingw64/include/c++/14.1.0/cstdlib:41,
                 from C:/msys64/mingw64/include/c++/14.1.0/stdlib.h:36,
                 from C:/gitlab-runner/builds/rau-wEy7/1/<...>/build/win/_deps/fmt-src/fmt/posix.h:22,
                 from C:/gitlab-runner/builds/rau-wEy7/1/<...>/build/win/_deps/fmt-src/fmt/posix.cc:15:
C:/msys64/mingw64/include/c++/14.1.0/x86_64-w64-mingw32/bits/c++config.h:667:2: warning: #warning "__STRICT_ANSI__ seems to have been undefined; this is not supported" [-Wcpp]
  667 | #warning "__STRICT_ANSI__ seems to have been undefined; this is not supported"
      |  ^~~~~~~
In file included from C:/gitlab-runner/builds/rau-wEy7/1/apdu/pos/baseApp/build/win/_deps/fmt-src/fmt/format.h:37,
                 from C:/gitlab-runner/builds/rau-wEy7/1/apdu/pos/baseApp/build/win/_deps/fmt-src/fmt/posix.h:30:
C:/msys64/mingw64/include/c++/14.1.0/limits:2100:30: error: exponent has no digits
 2100 |         return __extension__ 0x1.0p-16382Q;
      |                              ^~~~~~
C:/msys64/mingw64/include/c++/14.1.0/limits:2114:30: error: exponent has no digits
 2114 |         return __extension__ 0x1.ffffffffffffffffffffffffffffp+16383Q;
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:/msys64/mingw64/include/c++/14.1.0/limits:2183:30: error: exponent has no digits
 2183 |         return __extension__ 0x1.0p-16494Q;
      |                              ^~~~~~
C:/msys64/mingw64/include/c++/14.1.0/limits: In static member function 'static constexpr __float128 std::numeric_limits<__float128>::min()':
C:/msys64/mingw64/include/c++/14.1.0/limits:2100:37: error: unable to find numeric literal operator 'operator""Q'
 2100 |         return __extension__ 0x1.0p-16382Q;
      |                                     ^~~~~~
C:/msys64/mingw64/include/c++/14.1.0/limits:2100:37: note: use '-fext-numeric-literals' to enable more built-in suffixes
C:/msys64/mingw64/include/c++/14.1.0/limits:2102:7: error: body of 'constexpr' function 'static constexpr __float128 std::numeric_limits<__float128>::min()' not a return-statement
 2102 |       }
      |       ^
C:/msys64/mingw64/include/c++/14.1.0/limits: In static member function 'static constexpr __float128 std::numeric_limits<__float128>::max()':
C:/msys64/mingw64/include/c++/14.1.0/limits:2114:64: error: unable to find numeric literal operator 'operator""Q'
 2114 |         return __extension__ 0x1.ffffffffffffffffffffffffffffp+16383Q;
      |                                                                ^~~~~~
C:/msys64/mingw64/include/c++/14.1.0/limits:2114:64: note: use '-fext-numeric-literals' to enable more built-in suffixes
C:/msys64/mingw64/include/c++/14.1.0/limits:2116:7: error: body of 'constexpr' function 'static constexpr __float128 std::numeric_limits<__float128>::max()' not a return-statement
 2116 |       }
      |       ^
C:/msys64/mingw64/include/c++/14.1.0/limits: In static member function 'static constexpr __float128 std::numeric_limits<__float128>::denorm_min()':
C:/msys64/mingw64/include/c++/14.1.0/limits:2183:37: error: unable to find numeric literal operator 'operator""Q'
 2183 |         return __extension__ 0x1.0p-16494Q;
      |                                     ^~~~~~
C:/msys64/mingw64/include/c++/14.1.0/limits:2183:37: note: use '-fext-numeric-literals' to enable more built-in suffixes
C:/msys64/mingw64/include/c++/14.1.0/limits:2185:7: error: body of 'constexpr' function 'static constexpr __float128 std::numeric_limits<__float128>::denorm_min()' not a return-statement
 2185 |       }
      |       ^

[vitaut]

fmt 4.x is not actively maintained but a PR to branch 4.x would be welcome.