Added support for subsecond resolution for time_point

include/fmt/chrono.h

@@ -412,7 +412,11 @@ struct formatter<std::chrono::time_point<std::chrono::system_clock, Duration>,
   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);
+    auto epoch = val.time_since_epoch();
+    auto seconds = std::chrono::duration_cast<std::chrono::milliseconds>(epoch).count() / 1000;
+    auto today = std::chrono::high_resolution_clock::duration(std::chrono::milliseconds(seconds * 1000));
+    auto subseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(epoch - today);
+    return formatter<std::tm, Char>::format(time, ctx, subseconds.count());
   }
 };
 
@@ -428,14 +432,20 @@ template <typename Char> struct formatter<std::tm, Char> {
   }
 
   template <typename FormatContext>
-  auto format(const std::tm& tm, FormatContext& ctx) const
+  auto format(const std::tm& tm, FormatContext& ctx, int subseconds) const
       -> decltype(ctx.out()) {
     basic_memory_buffer<Char> tm_format;
     tm_format.append(specs.begin(), specs.end());
     // By appending an extra space we can distinguish an empty result that
     // indicates insufficient buffer size from a guaranteed non-empty result
     // https://github.com/fmtlib/fmt/issues/2238
-    tm_format.push_back(' ');
+    auto hasS = std::find(tm_format.begin(), tm_format.end(), 'S') != tm_format.end();
+    auto hasT = std::find(tm_format.begin(), tm_format.end(), 'T') != tm_format.end();
+    auto writeSubseconds = (hasS || hasT) && (subseconds != 0);
+    if (writeSubseconds)
+      tm_format.push_back('.');
+    else
+      tm_format.push_back(' ');
     tm_format.push_back('\0');
     basic_memory_buffer<Char> buf;
     size_t start = buf.size();
@@ -449,6 +459,11 @@ template <typename Char> struct formatter<std::tm, Char> {
       const size_t MIN_GROWTH = 10;
       buf.reserve(buf.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
     }
+
+    if (writeSubseconds) {
+      buf.append(std::to_string(subseconds));
+    }
+
     // Remove the extra space.
     return std::copy(buf.begin(), buf.end() - 1, ctx.out());
   }