[gcc r13-5004] libstdc++: Support single components in name of chrono::current_zone() [PR108211]

[Jonathan Wakely]

https://gcc.gnu.org/g:56be1970765b6302de19977790a537d6feaaa34b

commit r13-5004-g56be1970765b6302de19977790a537d6feaaa34b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jan 4 20:49:59 2023 +0000

    libstdc++: Support single components in name of chrono::current_zone() [PR108211]
    
    We currently only handle the case where /etc/localtime is a symlink to a
    path like ".../Etc/UTC" and fail for ".../UTC". This makes both work.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/108211
            * src/c++20/tzdb.cc (chrono::current_zone()): Check for zone
            using only last component of the name.

Diff:
---
 libstdc++-v3/src/c++20/tzdb.cc | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 6772517d55a..9103b159400 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -1501,8 +1501,11 @@ namespace std::chrono
 	if (std::distance(first, last) > 2)
 	  {
 	    --last;
-	    string name = std::prev(last)->string() + '/';
-	    name += last->string();
+	    string name = last->string();
+	    if (auto tz = do_locate_zone(this->zones, this->links, name))
+	      return tz;
+	    --last;
+	    name = last->string() + '/' + name;
 	    if (auto tz = do_locate_zone(this->zones, this->links, name))
 	      return tz;
 	  }