Without this qualification I have this in _GLIBCXX_INLINE_VERSION mode:

/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/locale_facets.h:2649: 
note: candidate: 'template<class _CharT> bool std::__9::isxdigit(_CharT, 
const locale&)'
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/locale_facets.h:2649: 
note:   template argument deduction/substitution failed:
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/format:1540: 
note:   candidate expects 2 arguments, 1 provided
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/format:1630: 
error: no matching function for call to 'isxdigit(const 
std::__9::basic_string_view<char>::value_type&)'
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/locale_facets.h:2649: 
note: candidate: 'template<class _CharT> bool std::__9::isxdigit(_CharT, 
const locale&)'
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/locale_facets.h:2649: 
note:   template argument deduction/substitution failed:
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/format:1630: 
note:   candidate expects 2 arguments, 1 provided
compiler exited with status 1
FAIL: 17_intro/headers/c++2020/all_attributes.cc (test for excess errors)
Excess errors:
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/format:1540: 
error: no matching function for call to 'isxdigit(const 
std::__9::basic_string_view<char>::value_type&)'
/home/fdt/dev/gcc/build_versioned_ns/x86_64-pc-linux-gnu/libstdc++-v3/include/format:1630: 
error: no matching function for call to 'isxdigit(const 
std::__9::basic_string_view<char>::value_type&)'

It sounds like the most reasonable fix as this is how toupper is being 
called.

     libstdc++: Add missing std qualification on isxdigit calls

     libstdc++-v3/ChangeLog

             * include/std/format: Add std qualification on isxdigit calls.

Ok to commit ?

François