adding a default format for std::chrono::time_point<std::chrono::syst… (#2345)

include/fmt/chrono.h

@@ -451,14 +451,37 @@ FMT_END_DETAIL_NAMESPACE
 template <typename Char, typename Duration>
 struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
                  Char> : formatter<std::tm, Char> {
+  FMT_CONSTEXPR formatter() {
+    this->specs = {default_specs, sizeof(default_specs) / sizeof(Char)};
+  }
+
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    auto it = ctx.begin();
+    if (it != ctx.end() && *it == ':') ++it;
+    auto end = it;
+    while (end != ctx.end() && *end != '}') ++end;
+    if (end != it) this->specs = {it, detail::to_unsigned(end - it)};
+    return end;
+  }
+
   template <typename FormatContext>
   auto format(std::chrono::time_point<std::chrono::system_clock> val,
               FormatContext& ctx) -> decltype(ctx.out()) {
     std::tm time = localtime(val);
     return formatter<std::tm, Char>::format(time, ctx);
   }
+
+  static constexpr Char default_specs[] = {'%', 'Y', '-', '%', 'm', '-',
+                                           '%', 'd', ' ', '%', 'H', ':',
+                                           '%', 'M', ':', '%', 'S'};
 };
 
+template <typename Char, typename Duration>
+constexpr Char
+    formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
+              Char>::default_specs[];
+
 template <typename Char> struct formatter<std::tm, Char> {
   template <typename ParseContext>
   FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {

test/chrono-test.cc

@@ -99,6 +99,7 @@ template <typename TimePoint> auto strftime(TimePoint tp) -> std::string {
 TEST(chrono_test, time_point) {
   auto t1 = std::chrono::system_clock::now();
   EXPECT_EQ(strftime(t1), fmt::format("{:%Y-%m-%d %H:%M:%S}", t1));
+  EXPECT_EQ(strftime(t1), fmt::format("{}", t1));
   using time_point =
       std::chrono::time_point<std::chrono::system_clock, std::chrono::seconds>;
   auto t2 = time_point(std::chrono::seconds(42));

test/compile-test.cc

@@ -92,6 +92,8 @@ TEST(compile_test, format_default) {
   EXPECT_EQ("foo", fmt::format(FMT_COMPILE("{}"), "foo"));
   EXPECT_EQ("foo", fmt::format(FMT_COMPILE("{}"), std::string("foo")));
   EXPECT_EQ("foo", fmt::format(FMT_COMPILE("{}"), test_formattable()));
+  auto t = std::chrono::system_clock::now();
+  EXPECT_EQ(fmt::format("{}", t), fmt::format(FMT_COMPILE("{}"), t));
 #  ifdef __cpp_lib_byte
   EXPECT_EQ("42", fmt::format(FMT_COMPILE("{}"), std::byte{42}));
 #  endif