Make ostream operators opt in to reduce the risk of ODR violations

include/fmt/core.h

@@ -1016,7 +1016,11 @@ struct fallback_formatter {
 // Specifies if T has an enabled fallback_formatter specialization.
 template <typename T, typename Char>
 using has_fallback_formatter =
+#ifdef FMT_DEPRECATED_OSTREAM
     std::is_constructible<fallback_formatter<T, Char>>;
+#else
+    std::false_type;
+#endif
 
 struct view {};
 

include/fmt/ostream.h

@@ -105,6 +105,9 @@ struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
 };
 }  // namespace detail
 
+template <typename T, typename Char = char>
+using ostream_formatter = detail::fallback_formatter<T, Char>;
+
 FMT_MODULE_EXPORT
 template <typename Char>
 void vprint(std::basic_ostream<Char>& os, basic_string_view<Char> format_str,