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

[Jonathan Wakely]

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.
---

I'm testing this now. Initial testing passed on x86_64-linux.

 libstdc++-v3/include/std/format                       | 11 +++++++++--
 libstdc++-v3/testsuite/std/format/functions/format.cc | 10 ++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index b4929d5ae59f..ec76ab0682e0 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2398,9 +2398,16 @@ namespace __format
        const size_t __r = __str.size() - __e; // Length of remainder.
        auto __overwrite = [&](_CharT* __p, size_t) {
          // Apply grouping to the digits before the radix or exponent.
-         auto __end = std::__add_grouping(__p, __np.thousands_sep(),
+         int __off = 0;
+         if (auto __c = __str.front(); __c == '-' || __c == '+' || __c == ' ')
+           {
+             *__p = __c;
+             __off = 1;
+           }
+         auto __end = std::__add_grouping(__p + __off, __np.thousands_sep(),
                                           __grp.data(), __grp.size(),
-                                          __str.data(), __str.data() + __e);
+                                          __str.data() + __off,
+                                          __str.data() + __e);
          if (__r) // If there's a fractional part or exponent
            {
              if (__d != __str.npos)
diff --git a/libstdc++-v3/testsuite/std/format/functions/format.cc 
b/libstdc++-v3/testsuite/std/format/functions/format.cc
index e4adf3aeb706..2af0bbeaa144 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}", -123.45);
+  VERIFY( s == "-123.45" );
+  s = std::format(bloc, "{:+L}", 123.45);
+  VERIFY( s == "+123.45" );
+  s = std::format(bloc, "{: L}",  123.45);
+  VERIFY( s == " 123.45" );
+
   // Restore
   std::locale::global(cloc);
 }
-- 
2.49.0