From 61097cc3d29764413e14bb9347fe715e5c85fc3e Mon Sep 17 00:00:00 2001
From: timsong-cpp <rs2740@gmail.com>
Date: Thu, 17 Mar 2022 23:53:48 -0500
Subject: [PATCH] Fix #2816: strip named argument wrappers for compile-time
 checking

---
 include/fmt/core.h  | 13 ++++++++++++-
 test/format-test.cc |  3 +++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 1c81bbd2d9804..4cb09bed6622d 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -2647,6 +2647,16 @@ FMT_CONSTEXPR FMT_INLINE void parse_format_string(
   }
 }
 
+template<class T, bool = is_named_arg<T>::value>
+struct strip_named_arg {
+  using type = T;
+};
+
+template<class T>
+struct strip_named_arg<T, true> {
+  using type = remove_cvref_t<decltype(T::value)>;
+};
+
 template <typename T, typename ParseContext>
 FMT_CONSTEXPR auto parse_format_specs(ParseContext& ctx)
     -> decltype(ctx.begin()) {
@@ -2654,7 +2664,8 @@ FMT_CONSTEXPR auto parse_format_specs(ParseContext& ctx)
   using context = buffer_context<char_type>;
   using mapped_type = conditional_t<
       mapped_type_constant<T, context>::value != type::custom_type,
-      decltype(arg_mapper<context>().map(std::declval<const T&>())), T>;
+      decltype(arg_mapper<context>().map(std::declval<const T&>())),
+      typename strip_named_arg<T>::type>;
   auto f = conditional_t<has_formatter<mapped_type, context>::value,
                          formatter<mapped_type, char_type>,
                          fallback_formatter<T, char_type>>();
diff --git a/test/format-test.cc b/test/format-test.cc
index e7f65daf32751..fc57dde325085 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1816,6 +1816,7 @@ TEST(format_test, compile_time_string) {
                                   "foo"_a = "foo"));
   EXPECT_EQ("", fmt::format(FMT_STRING("")));
   EXPECT_EQ("", fmt::format(FMT_STRING(""), "arg"_a = 42));
+  EXPECT_EQ("42", fmt::format(FMT_STRING("{answer}"), "answer"_a=Answer()));
 #endif
 
   (void)static_with_null;
@@ -1885,6 +1886,8 @@ TEST(format_test, named_arg_udl) {
       fmt::format("{first}{second}{first}{third}", fmt::arg("first", "abra"),
                   fmt::arg("second", "cad"), fmt::arg("third", 99)),
       udl_a);
+
+  EXPECT_EQ("42", fmt::format("{answer}", "answer"_a=Answer()));
 }
 #endif  // FMT_USE_USER_DEFINED_LITERALS