Fix compile-time checks for user-defined types (#1292)

include/fmt/core.h

@@ -207,6 +207,8 @@ template <typename T>
 using remove_reference_t = typename std::remove_reference<T>::type;
 template <typename T>
 using remove_const_t = typename std::remove_const<T>::type;
+template <typename T>
+using remove_cvref_t = typename std::remove_cv<remove_reference_t<T>>::type;
 
 struct monostate {};
 

include/fmt/format.h

@@ -3466,7 +3466,7 @@ template <typename Char, Char... CHARS> class udl_formatter {
   std::basic_string<Char> operator()(Args&&... args) const {
     FMT_CONSTEXPR_DECL Char s[] = {CHARS..., '\0'};
     FMT_CONSTEXPR_DECL bool invalid_format =
-        do_check_format_string<Char, error_handler, Args...>(
+        do_check_format_string<Char, error_handler, remove_cvref_t<Args>...>(
             basic_string_view<Char>(s, sizeof...(CHARS)));
     (void)invalid_format;
     return format(s, std::forward<Args>(args)...);

test/format-test.cc

@@ -1894,6 +1894,11 @@ TEST(FormatTest, UdlTemplate) {
   EXPECT_EQ("foo", "foo"_format());
   EXPECT_EQ("        42", "{0:10}"_format(42));
 }
+
+TEST(FormatTest, UdlPassUserDefinedObjectAsLvalue) {
+  Date date(2015, 10, 21);
+  EXPECT_EQ("2015-10-21", "{}"_format(date));
+}
 #endif  // FMT_USE_USER_DEFINED_LITERALS
 
 enum TestEnum { A };