Fix FMT_COMPILE failure with format_as mapped types (#4794)

include/fmt/compile.h

@@ -168,7 +168,13 @@ constexpr auto get_arg_checked(const Args&... args) -> const T& {
 template <typename Char>
 struct is_compiled_format<code_unit<Char>> : std::true_type {};
 
-// A replacement field that refers to argument N.
+
+template <typename T, typename = void>
+struct compile_has_format_as : std::false_type {};
+
+template <typename T>
+struct compile_has_format_as<T, void_t<decltype(format_as(std::declval<const T&>()))>> : std::true_type {};
+
 template <typename Char, typename V, int N> struct field {
   using char_type = Char;
 
@@ -178,6 +184,8 @@ template <typename Char, typename V, int N> struct field {
     if constexpr (std::is_convertible<V, basic_string_view<Char>>::value) {
       auto s = basic_string_view<Char>(arg);
       return copy<Char>(s.begin(), s.end(), out);
+    } else if constexpr (compile_has_format_as<V>::value) {
+      return write<Char>(out, format_as(arg));
     } else {
       return write<Char>(out, arg);
     }

test/compile-test.cc

@@ -479,3 +479,17 @@ TEST(compile_test, constexpr_string_format) {
   EXPECT_TRUE(big);
 }
 #endif  // FMT_USE_CONSTEXPR_STRING
+
+
+
+
+namespace {
+struct CompileFormatAsType {
+  int value;
+};
+int format_as(CompileFormatAsType f) { return f.value; }
+} // namespace
+
+TEST(CompileTest, FormatAs) {
+  EXPECT_EQ("42", fmt::format(FMT_COMPILE("{}"), CompileFormatAsType{42}));
+}