[gcc r15-6317] libstdc++: Call regex_traits::transform_primary() only when necessary [PR98723]

[Jonathan Wakely]

https://gcc.gnu.org/g:eb339c29ee42aa59591fc50d6d8a1ab903d2a3fe

commit r15-6317-geb339c29ee42aa59591fc50d6d8a1ab903d2a3fe
Author: Luca Bacci <luca.bacci982@gmail.com>
Date:   Tue Dec 17 18:57:30 2024 +0000

    libstdc++: Call regex_traits::transform_primary() only when necessary [PR98723]
    
    This is both a performance optimization and a partial fix for PR 98723.
    
    This commit fixes the issue for bracket expressions that do not depend
    on the locale's collation facet. Examples:
    
     * Character ranges ([a-z]) when std::regex::collate is not set
     * Character classes ([:alnum:])
     * Individual characters ([abc])
    
    Signed-off-by: Luca Bacci <luca.bacci982@gmail.com>
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/98723
            * include/bits/regex_compiler.tcc (_BracketMatcher::_M_apply):
            Only use transform_primary when an equivalence set is used.

Diff:
---
 libstdc++-v3/include/bits/regex_compiler.tcc | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/regex_compiler.tcc b/libstdc++-v3/include/bits/regex_compiler.tcc
index 9c5a46a47cd0..5037195ca17a 100644
--- a/libstdc++-v3/include/bits/regex_compiler.tcc
+++ b/libstdc++-v3/include/bits/regex_compiler.tcc
@@ -611,10 +611,13 @@ namespace __detail
 	    return true;
 	if (_M_traits.isctype(__ch, _M_class_set))
 	  return true;
-	if (std::find(_M_equiv_set.begin(), _M_equiv_set.end(),
-		      _M_traits.transform_primary(&__ch, &__ch+1))
-	    != _M_equiv_set.end())
-	  return true;
+	if (!_M_equiv_set.empty())
+	  {
+	    auto __x = _M_traits.transform_primary(&__ch, &__ch+1);
+	    auto __p = std::find(_M_equiv_set.begin(), _M_equiv_set.end(), __x);
+	    if (__p != _M_equiv_set.end())
+	      return true;
+	  }
 	for (auto& __it : _M_neg_class_set)
 	  if (!_M_traits.isctype(__ch, __it))
 	    return true;