Fix a UB in chrono

include/fmt/chrono.h

@@ -1663,7 +1663,7 @@ struct chrono_formatter {
     out = format_decimal<char_type>(out, n, num_digits).end;
   }
 
-  template <class Duration> void write_fractional_seconds(Duration d) {
+  template <typename Duration> void write_fractional_seconds(Duration d) {
     constexpr auto num_fractional_digits =
         count_fractional_digits(Duration::period::num, Duration::period::den);
 
@@ -1770,8 +1770,22 @@ struct chrono_formatter {
     if (handle_nan_inf()) return;
 
     if (ns == numeric_system::standard) {
-      write(second(), 2);
-      write_fractional_seconds(std::chrono::duration<rep, Period>{val});
+      if (std::is_floating_point<rep>::value) {
+        auto num_fractional_digits =
+            count_fractional_digits(Period::num, Period::den);
+        auto buf = memory_buffer();
+        format_to(std::back_inserter(buf), runtime("{:.{}f}"),
+                  std::fmod(val * static_cast<rep>(Period::num) /
+                                static_cast<rep>(Period::den),
+                            60),
+                  num_fractional_digits);
+        if (negative) *out++ = '-';
+        if (buf.size() < 2 || buf[1] == '.') *out++ = '0';
+        out = std::copy(buf.begin(), buf.end(), out);
+      } else {
+        write(second(), 2);
+        write_fractional_seconds(std::chrono::duration<rep, Period>(val));
+      }
       return;
     }
     auto time = tm();

test/chrono-test.cc

@@ -557,6 +557,9 @@ TEST(chrono_test, special_durations) {
             "03:33");
   EXPECT_EQ(fmt::format("{:%T}", std::chrono::duration<char, std::mega>{2}),
             "03:33:20");
+  EXPECT_EQ("44.000000000000",
+            fmt::format("{:%S}", std::chrono::duration<float, std::pico>(
+                                     1.54213895E+26)));
 }
 
 TEST(chrono_test, unsigned_duration) {