[libc++][chrono] Fixes formatting duration subseconds.

[Mordante]

Summary

Fixes https://llvm.org/PR62082

[Mordante]

Fixes CI.

[ldionne]

Thanks!

libcxx/include/__chrono/formatter.h

if constexpr (chrono::treat_as_floating_point_v<_Rep>)
     //   floating-point decimal seconds cannot be made within 18 fractional
     //   digits).
     //
     // This matches the behaviour of MSVC STL, fmtlib interprets this
     // differently and uses 3 decimals.
     // https://godbolt.org/z/6dsbnW8ba
     std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
                    _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}.0f}"),
-                   __fraction.count(),
+                   chrono::duration_cast<typename chrono::hh_mm_ss<__duration>::precision>(__fraction).count(),
                    chrono::hh_mm_ss<__duration>::fractional_width);
   else
     std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
                    _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}}"),
-                   __fraction.count(),
+                   chrono::duration_cast<typename chrono::hh_mm_ss<__duration>::precision>(__fraction).count(),
                    chrono::hh_mm_ss<__duration>::fractional_width);
 }
 
 template <class _CharT, __is_time_point _Tp>
 _LIBCPP_HIDE_FROM_ABI void __format_sub_seconds(const _Tp& __value, basic_stringstream<_CharT>& __sstr) {
   __formatter::__format_sub_seconds(__value.time_since_epoch(), __sstr);
 }
 

libcxx/test/std/time/time.syn/formatter.duration.pass.cpp

 template <class CharT>
 static void test_valid_values() {
   test_valid_positive_integral_values<CharT>();
   test_valid_negative_integral_values<CharT>();
   test_valid_fractional_values<CharT>();
 }
 
 template <class CharT>
+static void test_pr62082() {
+  // Examples in https://llvm.org/PR62082
+  check(SV("39.223300"), SV("{:%S}"), std::chrono::duration<int, std::ratio<101, 103>>{40});
+  check(SV("01.4755859375"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 1024>>{1511});
+
+  // Test with all possible number of decimals [0, 18]. When it does not
+  // fit in 18 decimals it uses 6.
+  check(SV("05"), SV("{:%S}"), std::chrono::duration<float, std::ratio<1, 1>>{10}); // 0
+
+  check(SV("05.0"), SV("{:%S}"), std::chrono::duration<float, std::ratio<1, 2>>{10}); // 1
+  check(SV("05.5"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 2>>{11});   // 1
+
+  check(SV("01.00"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 4>>{4});         // 2
+  check(SV("01.50"), SV("{:%S}"), std::chrono::duration<double, std::ratio<1, 4>>{6});      // 2
+  check(SV("01.75"), SV("{:%S}"), std::chrono::duration<long double, std::ratio<1, 4>>{7}); // 2
+
+  check(SV("01.000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 8>>{8});                          // 3
+  check(SV("01.0000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 16>>{16});                       // 4
+  check(SV("01.00000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 32>>{32});                      // 5
+  check(SV("01.000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 64>>{64});                     // 6
+  check(SV("01.0000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 128>>{128});                  // 7
+  check(SV("01.00000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 256>>{256});                 // 8
+  check(SV("01.000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 512>>{512});                // 9
+  check(SV("01.0000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 1024>>{1024});             // 10
+  check(SV("01.00000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 2048>>{2048});            // 11
+  check(SV("01.000000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 4096>>{4096});           // 12
+  check(SV("01.0000000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 8192>>{8192});          // 13
+  check(SV("01.00000000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 16384>>{16384});       // 14
+  check(SV("01.000000000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 32768>>{32768});      // 15
+  check(SV("01.0000000000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 65536>>{65536});     // 16
+  check(SV("01.00000000000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 131072>>{131072});  // 17
+  check(SV("01.000000000000000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 262144>>{262144}); // 18
+  check(SV("01.000000"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 524288>>{524288});             // 19 -> 6
+
+  // Infinite number of decimals will use 6 decimals.
+  check(SV("00.111111"), SV("{:%S}"), std::chrono::duration<int, std::ratio<1, 9>>{1});
+  check(SV("00.111111"), SV("{:%S}"), std::chrono::duration<float, std::ratio<1, 9>>{1});
+  check(SV("00.111111"), SV("{:%S}"), std::chrono::duration<double, std::ratio<1, 9>>{1});
+  check(SV("00.111111"), SV("{:%S}"), std::chrono::duration<long double, std::ratio<1, 9>>{1});
+}
+
+template <class CharT>
 static void test() {
   using namespace std::literals::chrono_literals;
 
   test_no_chrono_specs<CharT>();
   test_valid_values<CharT>();
+  test_pr62082<CharT>();
+
   check_invalid_types<CharT>(
       {SV("H"), SV("I"), SV("j"), SV("M"), SV("n"), SV("O"),  SV("p"),  SV("q"),  SV("Q"),  SV("r"),
        SV("R"), SV("S"), SV("t"), SV("T"), SV("X"), SV("EX"), SV("OH"), SV("OI"), SV("OM"), SV("OS")},
       0ms);
 
   check_exception("Expected '%' or '}' in the chrono format-string", SV("{:A"), 0ms);
   check_exception("The chrono-specs contains a '{'", SV("{:%%{"), 0ms);
   check_exception("End of input while parsing the modifier chrono conversion-spec", SV("{:%"), 0ms);