Fix the default locale in ostream_formatter

test/ostream-test.cc

@@ -17,7 +17,7 @@ struct test {};
 // included after fmt/format.h.
 namespace fmt {
 template <> struct formatter<test> : formatter<int> {
-  auto format(const test&, format_context& ctx) -> decltype(ctx.out()) {
+  auto format(const test&, format_context& ctx) const -> decltype(ctx.out()) {
     return formatter<int>::format(42, ctx);
   }
 };
@@ -289,3 +289,20 @@ TEST(ostream_test, closed_ofstream) {
   std::ofstream ofs;
   fmt::print(ofs, "discard");
 }
+
+struct unlocalized {};
+
+auto operator<<(std::ostream& os, unlocalized)
+    -> std::ostream& {
+  return os << 12345;
+}
+
+namespace fmt {
+template <> struct formatter<unlocalized> : ostream_formatter {};
+}  // namespace fmt
+
+TEST(ostream_test, unlocalized) {
+  auto loc = get_locale("en_US.UTF-8");
+  std::locale::global(loc);
+  EXPECT_EQ(fmt::format(loc, "{}", unlocalized()), "12345");
+}