Build failure using gcc 16.1.0

[xytovl]

Bug in Gentoo: https://bugs.gentoo.org/975268

gcc --version
gcc (Gentoo 16.1.0 p1) 16.1.0
Copyright (C) 2026 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

/tmp/KTX-Software-4.4.2.old/tools/ktx/command.h: In instantiation of 'void ktx::Reporter::fatal_usage(Args&& ...) [with Args = {const char (&)[26], const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]':
/tmp/KTX-Software-4.4.2.old/tools/ktx/command.h:233:31:   required from here
  233 |             report.fatal_usage("Unsupported format: \"{}\".", formatStr);
      |             ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/KTX-Software-4.4.2.old/tools/ktx/command.h:122:19: error: call to consteval function 'fmt::v11::fstring<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>((* & std::forward<const char (&)[26]>((* & args#0))))' is not a constant expression
  122 |         fmt::print(std::cerr, std::forward<Args>(args)...);
      |         ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/format.h:41,
                 from /tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/chrono.h:23,
                 from /tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/ostream.h:23,
                 from /tmp/KTX-Software-4.4.2.old/tools/ktx/utility.h:10,
                 from /tmp/KTX-Software-4.4.2.old/tools/ktx/command.h:12:
/tmp/KTX-Software-4.4.2.old/tools/ktx/command.h:122:19:   in 'constexpr' expansion of 'fmt::v11::fstring<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>((* & std::forward<const char (&)[26]>((* & args#0))))'
/tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/base.h:2736:53:   in 'constexpr' expansion of 'fmt::v11::basic_string_view<char>(((const char*)s))'
 2736 |     if (FMT_USE_CONSTEVAL) parse_format_string<char>(s, checker(s, arg_pack()));
      |                            ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/base.h:548:5: error: 'args#0' is not a constant expression
  548 |     while (*s++) ++len;
      |     ^~~~~

The following code no longer builds with GCC 16.1:

template <typename ...Arg>  
inline void do_print(Arg&&...a )  
{                       
        fmt::print(std::cerr, std::forward<Arg>(a)...);                                            
}

It can be fixed with

template <typename ...Arg>                               
inline void do_print(fmt::format_string<Arg...> f, Arg&&...a )  
{                                                                                                                                                                                          
        fmt::print(std::cerr, f, std::forward<Arg>(a)...);                                                                                                                                 
} 

This is however not sufficient and we get other fmt related failures in formatOptionalFileOffset, DiffHexFixedWidth.

I am not sure if this is a bug in KTX-Software, libfmt or gcc.
Replacing the bundled libfmt with system one 12.1.0 does not fix the issue.