libstdc++: Fix width computation for the chrono formatting [PR120114]

Use `__unicode::_field_width` to compute the field width of the output when writting
the formatted output for std::chrono::types. This applies both to characters copied
from format string, and one produced by localized formatting.

We also use _Str_sink::view() instead of get(), which avoids copying the content of
the buffer to std::string in case of small output.

	PR libstdc++/120114

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_format): Use __field_width.
	* testsuite/std/time/format/pr120114.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

1 files changed, 125 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/std/time/format/pr120114.cc b/libstdc++-v3/testsuite/std/time/format/pr120114.cc
new file mode 100644
index 0000000..c630bb3
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/format/pr120114.cc
@@ -0,0 +1,125 @@
+// { dg-do run { target c++23 } }
+// { dg-options "-fexec-charset=UTF-8" }
+// { dg-timeout-factor 2 }
+
+#include <algorithm>
+#include <chrono>
+#include <testsuite_hooks.h>
+
+#define WIDEN_(C, S) ::std::__format::_Widen<C>(S, L##S)
+#define WIDEN(S) WIDEN_(_CharT, S)
+
+template<typename _CharT>
+void
+test_from_format_string()
+{
+  std::basic_string<_CharT> res;
+  using namespace std::chrono_literals;
+  auto date = 2025y/std::chrono::May/05d;
+
+  res = std::format(WIDEN("{:+<13%F\U0001f921}"), date);
+  VERIFY( res == WIDEN("2025-05-05\U0001f921+") );
+
+  res = std::format(WIDEN("{:->15%F\U0001f921}"), date);
+  VERIFY( res == WIDEN("---2025-05-05\U0001f921") );
+
+  res = std::format(WIDEN("{:=^20%F\U0001f921}"), date);
+  VERIFY( res == WIDEN("====2025-05-05\U0001f921====") );
+}
+
+template<typename _CharT>
+void
+test_formatted_value()
+{
+  // Custom time_put facet which returns Ideographic Telegraph Symbol
+  // for given month for Om.
+  struct TimePut : std::time_put<_CharT>
+  {
+    using iter_type = std::time_put<_CharT>::iter_type;
+    using char_type = std::time_put<_CharT>::char_type;
+
+    iter_type
+    do_put(iter_type out, std::ios_base& io, char_type fill, const tm* t,
+	   char format, char modifier) const override
+    {
+      if (format != 'm' && modifier != 'm')
+	return std::time_put<_CharT>::do_put(out, io, fill, t, format, modifier);
+      std::basic_string_view<_CharT> str;
+      switch (t->tm_mon)
+       {
+	 case 0:
+	   str = WIDEN("\u32C0");
+	   break;
+	 case 1:
+	   str = WIDEN("\u32C1");
+	   break;
+	 case 2:
+	   str = WIDEN("\u32C2");
+	   break;
+	 case 3:
+	   str = WIDEN("\u32C3");
+	   break;
+	 case 4:
+	   str = WIDEN("\u32C4");
+	   break;
+	 case 5:
+	   str = WIDEN("\u32C5");
+	   break;
+	 case 6:
+	   str = WIDEN("\u32C6");
+	   break;
+	 case 7:
+	   str = WIDEN("\u32C7");
+	   break;
+	 case 8:
+	   str = WIDEN("\u32C8");
+	   break;
+	 case 9:
+	   str = WIDEN("\u32C9");
+	   break;
+	 case 10:
+	   str = WIDEN("\u32CA");
+	   break;
+	 case 11:
+	   str = WIDEN("\u32CB");
+	   break;
+       };
+       return std::copy(str.begin(), str.end(), out);
+    }
+  };
+  const std::locale loc(std::locale::classic(), new TimePut);
+
+  std::basic_string<_CharT> res;
+
+  res = std::format(loc, WIDEN("{:<1L%Om}"), std::chrono::January);
+  VERIFY( res == WIDEN("\u32C0") );
+
+  res = std::format(loc, WIDEN("{:>2L%Om}"), std::chrono::February);
+  VERIFY( res == WIDEN("\u32C1") );
+
+  res = std::format(loc, WIDEN("{:<3L%Om}"), std::chrono::March);
+  VERIFY( res == WIDEN("\u32C2 ") );
+
+  res = std::format(loc, WIDEN("{:^4L%Om}"), std::chrono::April);
+  VERIFY( res == WIDEN(" \u32C3 ") );
+
+  res = std::format(loc, WIDEN("{:>5L%Om}"), std::chrono::May);
+  VERIFY( res == WIDEN("   \u32C4") );
+
+  res = std::format(loc, WIDEN("{:+<6L%Om}"), std::chrono::June);
+  VERIFY( res == WIDEN("\u32C5++++") );
+
+  res = std::format(loc, WIDEN("{:=^7L%Om}"), std::chrono::July);
+  VERIFY( res == WIDEN("==\u32C6===") );
+
+  res = std::format(loc, WIDEN("{:->8L%Om}"), std::chrono::August);
+  VERIFY( res == WIDEN("------\u32C7") );
+}
+
+int main()
+{
+  test_from_format_string<char>();
+  test_from_format_string<wchar_t>();
+  test_formatted_value<char>();
+  test_formatted_value<wchar_t>();
+}