Add fmt::assert_fail as a weak symbol

That way one can provide ones own implementation for
fmt::assert_fail. Keeping the fmt::detail::assert_fail for binary
compatibility, which now just calls fmt::assert_fail.

Author: HazardyKnusperkeks

include/fmt/base.h

@@ -270,6 +270,16 @@ FMT_PRAGMA_CLANG(diagnostic push)
 #  define FMT_WIN32 0
 #endif
 
+#if (FMT_GCC_VERSION || FMT_CLANG_VERSION) && !defined(FMT_HEADER_ONLY) && \
+    !FMT_WIN32
+// Header only (which implies everything is inline) and weak symbols do not work
+// together. And on Windows weak symbols seem not to work properly.
+#  define FMT_WEAK __attribute__((weak))
+#endif
+#ifndef FMT_WEAK
+#  define FMT_WEAK
+#endif
+
 #if !defined(FMT_HEADER_ONLY) && FMT_WIN32
 #  if defined(FMT_LIB_EXPORT)
 #    define FMT_API __declspec(dllexport)
@@ -348,6 +358,11 @@ template <typename T> constexpr auto max_of(T a, T b) -> T {
   return a > b ? a : b;
 }
 
+FMT_BEGIN_EXPORT
+FMT_NORETURN FMT_API FMT_WEAK void assert_fail(const char* file, int line,
+                                               const char* message);
+FMT_END_EXPORT
+
 namespace detail {
 // Suppresses "unused variable" warnings with the method described in
 // https://herbsutter.com/2009/10/18/mailbag-shutting-up-compiler-warnings/.
@@ -388,7 +403,7 @@ FMT_NORETURN FMT_API void assert_fail(const char* file, int line,
 #  define FMT_ASSERT(condition, message)                                    \
     ((condition) /* void() fails with -Winvalid-constexpr on clang 4.0.1 */ \
          ? (void)0                                                          \
-         : fmt::detail::assert_fail(__FILE__, __LINE__, (message)))
+         : ::fmt::assert_fail(__FILE__, __LINE__, (message)))
 #endif
 
 #ifdef FMT_USE_INT128

include/fmt/format-inl.h

@@ -31,15 +31,22 @@
 #endif
 
 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);
   abort();
 }
 
+namespace detail {
+
+// For binary compatibility.
+FMT_FUNC void assert_fail(const char* file, int line, const char* message) {
+  ::fmt::assert_fail(file, line, message);
+}
+
 FMT_FUNC void format_error_code(detail::buffer<char>& out, int error_code,
                                 string_view message) noexcept {
   // Report error code making sure that the output fits into

include/fmt/format.h

@@ -183,8 +183,7 @@ FMT_END_NAMESPACE
 #      define FMT_THROW(x) throw x
 #    endif
 #  else
-#    define FMT_THROW(x) \
-      ::fmt::detail::assert_fail(__FILE__, __LINE__, (x).what())
+#    define FMT_THROW(x) ::fmt::assert_fail(__FILE__, __LINE__, (x).what())
 #  endif  // FMT_USE_EXCEPTIONS
 #endif    // FMT_THROW