Fix %S formatting for chrono durations with leading zeroes (#3814)

Author: js324

include/fmt/chrono.h

@@ -1151,18 +1151,23 @@ void write_fractional_seconds(OutputIt& out, Duration d, int precision = -1) {
       out = std::fill_n(out, leading_zeroes, '0');
       out = format_decimal<Char>(out, n, num_digits).end;
     }
-  } else {
+  } else if (precision > 0) {
     *out++ = '.';
     leading_zeroes = (std::min)(leading_zeroes, precision);
-    out = std::fill_n(out, leading_zeroes, '0');
     int remaining = precision - leading_zeroes;
-    if (remaining != 0 && remaining < num_digits) {
-      n /= to_unsigned(detail::pow10(to_unsigned(num_digits - remaining)));
-      out = format_decimal<Char>(out, n, remaining).end;
+    out = std::fill_n(out, leading_zeroes, '0');
+    if (remaining < num_digits) {
+      int num_truncated_digits = num_digits - remaining;
+      n /= to_unsigned(detail::pow10(to_unsigned(num_truncated_digits)));
+      if (n) {
+        out = format_decimal<Char>(out, n, remaining).end;
+      }
       return;
     }
-    out = format_decimal<Char>(out, n, num_digits).end;
-    remaining -= num_digits;
+    if (n) {
+      out = format_decimal<Char>(out, n, num_digits).end;
+      remaining -= num_digits;
+    }
     out = std::fill_n(out, remaining, '0');
   }
 }

test/chrono-test.cc

@@ -791,6 +791,20 @@ TEST(chrono_test, cpp20_duration_subsecond_support) {
             "01.234000");
   EXPECT_EQ(fmt::format("{:.6%S}", std::chrono::milliseconds{-1234}),
             "-01.234000");
+  EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{12345}),
+            "12.34");
+  EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{12375}),
+            "12.37");
+  EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{-12375}),
+            "-12.37");
+  EXPECT_EQ(fmt::format("{:.0%S}", std::chrono::milliseconds{12054}),
+            "12");
+  EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{99999}),
+            "39.99");
+  EXPECT_EQ(fmt::format("{:.2%S}", std::chrono::milliseconds{1000}),
+            "01.00");
+  EXPECT_EQ(fmt::format("{:.3%S}", std::chrono::milliseconds{1}),
+            "00.001");
   EXPECT_EQ(fmt::format("{:.3%S}", std::chrono::seconds{1234}), "34.000");
   EXPECT_EQ(fmt::format("{:.3%S}", std::chrono::hours{1234}), "00.000");
   EXPECT_EQ(fmt::format("{:.5%S}", dms(1.234)), "00.00123");