With #4477 the usage of malloc and free was changed to std::malloc and std::free.
While this is most likely the correct approach, it breaks global allocator customization since the std:: variants can not be overriden.
One example would be mimalloc, which redefines these globally.

When such a library or global override is used, you end up with a compilation failure since the "free" in "std::free" is redefined to something else.

An alternative to reverting to global malloc/free could also be the introduction of defines that can be configured along the lines of this:

#ifndef FMT_MALLOC
#define FMT_MALLOC std::malloc
#endif

#ifndef FMT_FREE
#define FMT_FREE std::free
#endif