libstdc++: Fix formatting of chrono::duration with character rep [PR116755]

Implement Peter Dimov's suggestion for resolving LWG 4118, which is to
use +d.count() so that character types are promoted to an integer type
before formatting them. This didn't have unanimous consensus in the
committee as Howard Hinnant proposed that we should format the rep
consistently with std::format("{}", d.count()) instead. That ends up
being more complicated, because it makes std::formattable a precondition
of operator<< which was not previously the case, and it means that
ios_base::fmtflags from the stream would be ignored because std::format
doesn't use them.

libstdc++-v3/ChangeLog:

	PR libstdc++/116755
	* include/bits/chrono_io.h (operator<<): Use +d.count() for
	duration inserter.
	(__formatter_chrono::_M_format): Likewise for %Q format.
	* testsuite/20_util/duration/io.cc: Test durations with
	character types as reps.

2 files changed, 73 insertions, 2 deletions

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index 362bb5a..a337007 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -150,7 +150,9 @@ namespace __detail
       __s.flags(__os.flags());
       __s.imbue(__os.getloc());
       __s.precision(__os.precision());
-      __s << __d.count();
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 4118. How should duration formatters format custom rep types?
+      __s << +__d.count();
       __detail::__fmt_units_suffix<period, _CharT>(_Out(__s));
       __os << std::move(__s).str();
       return __os;
@@ -635,8 +637,10 @@ namespace __format
 		case 'Q':
 		  // %Q The duration's numeric value.
 		  if constexpr (chrono::__is_duration_v<_Tp>)
+		    // _GLIBCXX_RESOLVE_LIB_DEFECTS
+		    // 4118. How should duration formatters format custom rep?
 		    __out = std::format_to(__print_sign(), _S_empty_spec,
-					   __t.count());
+					   +__t.count());
 		  else
 		    __throw_format_error("chrono format error: argument is "
 					 "not a duration");
@@ -1703,6 +1707,7 @@ namespace __format
 /// @endcond
 
   template<typename _Rep, typename _Period, typename _CharT>
+    requires __format::__formattable_impl<_Rep, _CharT>
     struct formatter<chrono::duration<_Rep, _Period>, _CharT>
     {
       constexpr typename basic_format_parse_context<_CharT>::iterator
diff --git a/libstdc++-v3/testsuite/20_util/duration/io.cc b/libstdc++-v3/testsuite/20_util/duration/io.cc
index 57020f4..383fb60 100644
--- a/libstdc++-v3/testsuite/20_util/duration/io.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/io.cc
@@ -23,6 +23,24 @@ test01()
   VERIFY( s == "3[2]s" );
   std::getline(ss, s);
   VERIFY( s == "9[2/3]s" );
+
+  // LWG 4118. How should duration formatters format custom rep types?
+  ss.str("");
+  ss << duration<char>(121) << ' ';
+  ss << duration<wchar_t>(122) << ' ';
+  ss << duration<signed char>(123) << ' ';
+  ss << duration<unsigned char>(124) << ' ';
+  ss << duration<char8_t>(125) << ' ';
+  ss << duration<char16_t>(126) << ' ';
+  ss << duration<char32_t>(127) << ' ';
+  VERIFY( ss.str() == "121s 122s 123s 124s 125s 126s 127s " );
+
+  ss.str("");
+  ss << std::hex << std::uppercase << duration<const char>(0x1A) << ' ';
+  ss << std::hex << std::uppercase << duration<const wchar_t>(0x2A) << ' ';
+  ss << std::hex << std::uppercase << duration<signed char>(0x3A) << ' ';
+  ss << std::scientific << duration<const double>(4.5) << ' ';
+  VERIFY( ss.str() == "1As 2As 3As 4.500000E+00s " );
 }
 
 void
@@ -44,6 +62,24 @@ test02()
   VERIFY( s == L"3[2]s" );
   std::getline(ss, s);
   VERIFY( s == L"9[2/3]s" );
+
+  // LWG 4118. How should duration formatters format custom rep types?
+  ss.str(L"");
+  ss << duration<char>(121) << ' ';
+  ss << duration<wchar_t>(122) << ' ';
+  ss << duration<signed char>(123) << ' ';
+  ss << duration<unsigned char>(124) << ' ';
+  ss << duration<char8_t>(125) << ' ';
+  ss << duration<char16_t>(126) << ' ';
+  ss << duration<char32_t>(127) << ' ';
+  VERIFY( ss.str() == L"121s 122s 123s 124s 125s 126s 127s " );
+
+  ss.str(L"");
+  ss << std::hex << std::uppercase << duration<const char>(0x1A) << ' ';
+  ss << std::hex << std::uppercase << duration<const wchar_t>(0x2A) << ' ';
+  ss << std::hex << std::uppercase << duration<signed char>(0x3A) << ' ';
+  ss << std::scientific << duration<const double>(4.5) << ' ';
+  VERIFY( ss.str() == L"1As 2As 3As 4.500000E+00s " );
 #endif
 }
 
@@ -114,6 +150,36 @@ test_format()
   VERIFY( s == expected );
   s = std::format("{:%Q%q}", minsec);
   VERIFY( s == expected );
+
+  // LWG 4118. How should duration formatters format custom rep types?
+  s = std::format("{}", std::chrono::duration<char>(100));
+  VERIFY( s == "100s" );
+  s = std::format("{:%Q}", std::chrono::duration<char>(101));
+  VERIFY( s == "101" );
+#ifdef _GLIBCXX_USE_WCHAR_T
+  ws = std::format(L"{}", std::chrono::duration<char>(102));
+  VERIFY( ws == L"102s" );
+  ws = std::format(L"{}", std::chrono::duration<wchar_t>(103));
+  VERIFY( ws == L"103s" );
+#endif
+  s = std::format("{}", std::chrono::duration<signed char>(50));
+  VERIFY( s == "50s" );
+  s = std::format("{:%Q}", std::chrono::duration<signed char>(51));
+  VERIFY( s == "51" );
+  s = std::format("{}", std::chrono::duration<unsigned char>(52));
+  VERIFY( s == "52s" );
+  s = std::format("{:%Q}", std::chrono::duration<unsigned char>(53));
+  VERIFY( s == "53" );
+
+#if __cplusplus > 202002L
+  static_assert( ! std::formattable<std::chrono::duration<wchar_t>, char> );
+  static_assert( ! std::formattable<std::chrono::duration<char8_t>, char> );
+  static_assert( ! std::formattable<std::chrono::duration<char16_t>, char> );
+  static_assert( ! std::formattable<std::chrono::duration<char32_t>, char> );
+  static_assert( ! std::formattable<std::chrono::duration<char8_t>, wchar_t> );
+  static_assert( ! std::formattable<std::chrono::duration<char16_t>, wchar_t> );
+  static_assert( ! std::formattable<std::chrono::duration<char32_t>, wchar_t> );
+#endif
 }
 
 void