Fix handling of mapped types in compile checks (#1200)

include/fmt/format.h

@@ -2096,10 +2096,14 @@ FMT_CONSTEXPR void parse_format_string(basic_string_view<Char> format_str,
 template <typename T, typename ParseContext>
 FMT_CONSTEXPR const typename ParseContext::char_type* parse_format_specs(
     ParseContext& ctx) {
-  // GCC 7.2 requires initializer.
-  typedef typename ParseContext::char_type char_type;
-  conditional_t<has_formatter<T, buffer_context<char_type>>::value,
-                formatter<T, char_type>,
+  using char_type = typename ParseContext::char_type;
+  using context = buffer_context<char_type>;
+  using mapped_type =
+      conditional_t<internal::mapped_type_constant<T, context>::value !=
+                        internal::custom_type,
+                    decltype(arg_mapper<context>().map(std::declval<T>())), T>;
+  conditional_t<has_formatter<mapped_type, context>::value,
+                formatter<mapped_type, char_type>,
                 internal::fallback_formatter<T, char_type>>
       f;
   return f.parse(ctx);

test/format-test.cc

@@ -1836,6 +1836,15 @@ TEST(FormatTest, UnpackedArgs) {
                         6, 7, 8, 9, 'a', 'b', 'c', 'd', 'e', 'f', 'g'));
 }
 
+struct string_like {};
+fmt::string_view to_string_view(string_like) { return "foo"; }
+
+TEST(FormatTest, CompileTimeString) {
+  EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), 42));
+  EXPECT_EQ(L"42", fmt::format(FMT_STRING(L"{}"), 42));
+  EXPECT_EQ("foo", fmt::format(FMT_STRING("{}"), string_like()));
+}
+
 #if FMT_USE_USER_DEFINED_LITERALS
 // Passing user-defined literals directly to EXPECT_EQ causes problems
 // with macro argument stringification (#) on some versions of GCC.
@@ -1867,8 +1876,6 @@ TEST(LiteralsTest, NamedArg) {
 TEST(FormatTest, UdlTemplate) {
   EXPECT_EQ("foo", "foo"_format());
   EXPECT_EQ("        42", "{0:10}"_format(42));
-  EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), 42));
-  EXPECT_EQ(L"42", fmt::format(FMT_STRING(L"{}"), 42));
 }
 #endif  // FMT_USE_USER_DEFINED_LITERALS