Suppress a warning

include/fmt/format.h

@@ -520,7 +520,7 @@ FMT_CONSTEXPR inline auto utf8_decode(const char* s, uint32_t* c, int* e)
   return next;
 }
 
-enum { invalid_code_point = ~uint32_t() };
+constexpr uint32_t invalid_code_point = ~uint32_t();
 
 // Invokes f(cp, sv) for every code point cp in s with sv being the string view
 // corresponding to the code point. cp is invalid_code_point on error.