Use overridden locale in ostream

include/fmt/core.h

@@ -1076,6 +1076,8 @@ class locale_ref {
   locale_ref() : locale_(nullptr) {}
   template <typename Locale> explicit locale_ref(const Locale& loc);
 
+  explicit operator bool() const FMT_NOEXCEPT { return locale_ != nullptr; }
+
   template <typename Locale> Locale get() const;
 };
 

include/fmt/ostream.h

@@ -89,9 +89,11 @@ void write(std::basic_ostream<Char>& os, buffer<Char>& buf) {
 }
 
 template <typename Char, typename T>
-void format_value(buffer<Char>& buf, const T& value) {
+void format_value(buffer<Char>& buf, const T& value,
+                  locale_ref loc = locale_ref()) {
   formatbuf<Char> format_buf(buf);
   std::basic_ostream<Char> output(&format_buf);
+  if (loc) output.imbue(loc.get<std::locale>());
   output.exceptions(std::ios_base::failbit | std::ios_base::badbit);
   output << value;
   buf.resize(buf.size());
@@ -104,7 +106,7 @@ struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
   template <typename Context>
   auto format(const T& value, Context& ctx) -> decltype(ctx.out()) {
     basic_memory_buffer<Char> buffer;
-    format_value(buffer, value);
+    format_value(buffer, value, ctx.locale());
     basic_string_view<Char> str(buffer.data(), buffer.size());
     return formatter<basic_string_view<Char>, Char>::format(str, ctx);
   }