Add formatters for chrono::time_point<system_clock> (#1837)

Add formatters for chrono::time_point and helper overloads for localtime/gmtime(time_point)
Fixes #1819

include/fmt/chrono.h

@@ -325,6 +325,11 @@ inline std::tm localtime(std::time_t time) {
   return lt.tm_;
 }
 
+inline std::tm localtime(
+    std::chrono::time_point<std::chrono::system_clock> time_point) {
+  return localtime(std::chrono::system_clock::to_time_t(time_point));
+}
+
 // Thread-safe replacement for std::gmtime
 inline std::tm gmtime(std::time_t time) {
   struct dispatcher {
@@ -361,6 +366,11 @@ inline std::tm gmtime(std::time_t time) {
   return gt.tm_;
 }
 
+inline std::tm gmtime(
+    std::chrono::time_point<std::chrono::system_clock> time_point) {
+  return gmtime(std::chrono::system_clock::to_time_t(time_point));
+}
+
 namespace detail {
 inline size_t strftime(char* str, size_t count, const char* format,
                        const std::tm* time) {
@@ -373,6 +383,17 @@ inline size_t strftime(wchar_t* str, size_t count, const wchar_t* format,
 }
 }  // namespace detail
 
+template <typename Char>
+struct formatter<std::chrono::time_point<std::chrono::system_clock>, Char>
+    : formatter<std::tm, Char> {
+  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);
+  }
+};
+
 template <typename Char> struct formatter<std::tm, Char> {
   template <typename ParseContext>
   auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {

test/chrono-test.cc

@@ -95,6 +95,17 @@ TEST(TimeTest, GMTime) {
   EXPECT_TRUE(EqualTime(tm, fmt::gmtime(t)));
 }
 
+TEST(TimeTest, TimePoint) {
+  std::chrono::system_clock::time_point point = std::chrono::system_clock::now();
+
+  std::time_t t = std::chrono::system_clock::to_time_t(point);
+  std::tm tm = *std::localtime(&t);
+  char strftime_output[256];
+  std::strftime(strftime_output, sizeof(strftime_output), "It is %Y-%m-%d %H:%M:%S", &tm);
+
+  EXPECT_EQ(strftime_output, fmt::format("It is {:%Y-%m-%d %H:%M:%S}", point));
+}
+
 #define EXPECT_TIME(spec, time, duration)                 \
   {                                                       \
     std::locale loc("ja_JP.utf8");                        \