[committed] libstdc++: Make std::println use locale from ostream (LWG 4088)

[Jonathan Wakely]

This was just approved in Wrocław.

libstdc++-v3/ChangeLog:

        * include/std/ostream (println): Pass stream's locale to
        std::format, as per LWG 4088.
        * testsuite/27_io/basic_ostream/print/1.cc: Check std::println
        with custom locale. Remove unused brit_punc class.
---

Tested x86_64-linux. Pushed to trunk.

 libstdc++-v3/include/std/ostream               |  6 ++++--
 .../testsuite/27_io/basic_ostream/print/1.cc   | 18 +++++++++---------
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/include/std/ostream b/libstdc++-v3/include/std/ostream
index 637aad5a5a4..327313a881d 100644
--- a/libstdc++-v3/include/std/ostream
+++ b/libstdc++-v3/include/std/ostream
@@ -1028,8 +1028,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline void
     println(ostream& __os, format_string<_Args...> __fmt, _Args&&... __args)
     {
-      std::print(__os, "{}\n",
-                std::format(__fmt, std::forward<_Args>(__args)...));
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 4088. println ignores the locale imbued in std::ostream
+      std::print(__os, "{}\n", std::format(__os.getloc(), __fmt,
+                                          std::forward<_Args>(__args)...));
     }
 
   // Defined for C++26, supported as an extension to C++23.
diff --git a/libstdc++-v3/testsuite/27_io/basic_ostream/print/1.cc 
b/libstdc++-v3/testsuite/27_io/basic_ostream/print/1.cc
index cd4b116ac1c..183e08733d2 100644
--- a/libstdc++-v3/testsuite/27_io/basic_ostream/print/1.cc
+++ b/libstdc++-v3/testsuite/27_io/basic_ostream/print/1.cc
@@ -63,14 +63,6 @@ test_vprint_nonunicode()
   // { dg-output "garbage in . garbage out" }
 }
 
-struct brit_punc : std::numpunct<char>
-{
-  std::string do_grouping() const override { return "\3\3"; }
-  char do_thousands_sep() const override { return ','; }
-  std::string do_truename() const override { return "yes mate"; }
-  std::string do_falsename() const override { return "nah bruv"; }
-};
-
 void
 test_locale()
 {
@@ -82,7 +74,7 @@ test_locale()
 
   // The default C locale.
   std::locale cloc = std::locale::classic();
-  // A custom locale using comma digit separators.
+  // A custom locale using tilde digit separators.
   std::locale bloc(cloc, new stream_punc);
 
   {
@@ -101,6 +93,14 @@ test_locale()
     std::print(os, "{:L} {}", 12345, 6789);
     VERIFY(os.str() == "1~23~45 6789");
   }
+
+  {
+    // LWG 4088. println ignores the locale imbued in std::ostream
+    std::ostringstream os;
+    os.imbue(bloc);
+    std::println(os, "{:L} {}", 12345, 6789);
+    VERIFY(os.str() == "1~23~45 6789\n");
+  }
 }
 
 void
-- 
2.47.1