Fix ABI compatiblity issue

include/fmt/core.h

@@ -615,8 +615,7 @@ struct error_handler {
   constexpr error_handler(const error_handler&) = default;
 
   // This function is intentionally not constexpr to give a compile-time error.
-  // This function is marked as FMT_API for backwards compatibility, see #2695.
-  FMT_NORETURN FMT_API void on_error(const char* message) { throw_format_error(message); }
+  FMT_NORETURN FMT_API void on_error(const char* message);
 };
 FMT_END_DETAIL_NAMESPACE
 

include/fmt/format-inl.h

@@ -2575,6 +2575,12 @@ FMT_FUNC void report_system_error(int error_code,
   report_error(format_system_error, error_code, message);
 }
 
+// DEPRECATED!
+// This function is defined here and not inline for ABI compatiblity.
+FMT_FUNC void detail::error_handler::on_error(const char* message) {
+  throw_format_error(message);
+}
+
 FMT_FUNC std::string vformat(string_view fmt, format_args args) {
   // Don't optimize the "{}" case to keep the binary size small and because it
   // can be better optimized in fmt::format anyway.