FMT_USE_USER_DEFINED_LITERALS has undefined behavior

[bylee20]

#ifndef FMT_USE_USER_DEFINED_LITERALS
// All compilers which support UDLs also support variadic templates. This
// makes the fmt::literals implementation easier. However, an explicit check
// for variadic templates is added here just in case.
// For Intel's compiler both it and the system gcc/msc must support UDLs.
# define FMT_USE_USER_DEFINED_LITERALS \
   FMT_USE_VARIADIC_TEMPLATES && FMT_USE_RVALUE_REFERENCES && \
   (FMT_HAS_FEATURE(cxx_user_literals) || \
     (FMT_GCC_VERSION >= 407 && FMT_HAS_GXX_CXX11) || FMT_MSC_VER >= 1900) && \
   (!defined(FMT_ICC_VERSION) || FMT_ICC_VERSION >= 1500)
#endif

Here, FMT_USE_USER_DEFINED_LITERALS contains defined, and clang produces warning for this:

...\fmt\format.h:3932: warning: macro expansion producing 'defined' has undefined behavior [-Wexpansion-to-defined]
#if FMT_USE_USER_DEFINED_LITERALS

This can be fixed, for instance:

#ifndef FMT_USE_USER_DEFINED_LITERALS
// All compilers which support UDLs also support variadic templates. This
// makes the fmt::literals implementation easier. However, an explicit check
// for variadic templates is added here just in case.
// For Intel's compiler both it and the system gcc/msc must support UDLs.
#if FMT_USE_VARIADIC_TEMPLATES && FMT_USE_RVALUE_REFERENCES && \
    (FMT_HAS_FEATURE(cxx_user_literals) || \
      (FMT_GCC_VERSION >= 407 && FMT_HAS_GXX_CXX11) || FMT_MSC_VER >= 1900) && \
    (!defined(FMT_ICC_VERSION) || FMT_ICC_VERSION >= 1500)
#define FMT_USE_USER_DEFINED_LITERALS 1
#else
#define FMT_USE_USER_DEFINED_LITERALS 0
#endif

[vitaut]

Good catch, could you by any chance submit a PR?

[bylee20]

@vitaut Sure: #542