libstdc++: Fix std::format thousands separators when sign present [PR120548]

The leading sign character should be skipped when deciding whether to
insert thousands separators into a floating-point format.

libstdc++-v3/ChangeLog:

	PR libstdc++/120548
	* include/std/format (__formatter_fp::_M_localize): Do not
	include a leading sign character in the string to be grouped.
	* testsuite/std/format/functions/format.cc: Check grouping when
	sign is present in the output.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/std/format/functions/format.cc b/libstdc++-v3/testsuite/std/format/functions/format.cc
index e4adf3a..d342114 100644
--- a/libstdc++-v3/testsuite/std/format/functions/format.cc
+++ b/libstdc++-v3/testsuite/std/format/functions/format.cc
@@ -261,6 +261,16 @@ test_locale()
   s = std::format(eloc, "{0:Le} {0:Lf} {0:Lg}", -nan);
   VERIFY( s == "-nan -nan -nan" );
 
+  // PR libstdc++/120548 format confuses a negative sign for a thousands digit
+  s = std::format(bloc, "{:L}", -123.45);
+  VERIFY( s == "-123.45" );
+  s = std::format(bloc, "{:-L}", -876543.21);
+  VERIFY( s == "-876,543.21" );
+  s = std::format(bloc, "{:+L}", 333.22);
+  VERIFY( s == "+333.22" );
+  s = std::format(bloc, "{: L}", 999.44);
+  VERIFY( s == " 999.44" );
+
   // Restore
   std::locale::global(cloc);
 }