Mark detail::assert_fail as weak symbol

That way one can provide ones own implementation.

Author: HazardyKnusperkeks

include/fmt/base.h

@@ -215,6 +215,12 @@
 #  define FMT_VISIBILITY(value)
 #endif
 
+#if FMT_GCC_VERSION || FMT_CLANG_VERSION
+#  define FMT_WEAK __attribute__((weak))
+#else
+#  define FMT_WEAK
+#endif
+
 // Detect pragmas.
 #define FMT_PRAGMA_IMPL(x) _Pragma(#x)
 #if FMT_GCC_VERSION >= 504 && !defined(__NVCOMPILER)

include/fmt/format-inl.h

@@ -33,7 +33,8 @@
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
-FMT_FUNC void assert_fail(const char* file, int line, const char* message) {
+FMT_FUNC FMT_WEAK void assert_fail(const char* file, int line,
+                                   const char* message) {
   // Use unchecked std::fprintf to avoid triggering another assertion when
   // writing to stderr fails.
   fprintf(stderr, "%s:%d: assertion failed: %s", file, line, message);