[gcc r15-3499] libstdc++: -Wswitch and ios::openmode

[Jason Merrill]

https://gcc.gnu.org/g:3dafb65bb5c31b169dae180e0664dfcaee64afe6

commit r15-3499-g3dafb65bb5c31b169dae180e0664dfcaee64afe6
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Aug 27 13:16:27 2024 -0400

    libstdc++: -Wswitch and ios::openmode
    
    In addition to marking it as flag_enum, we want to avoid warnings about
    not having a case for the implementation detail enumerators
    _S_ios_openmode_*.  And also for _S_noreplace in standard modes before it
    was added.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/ios_base.h (_GLIBCXX_NOREPLACE_UNUSED): New.
            (_Ios_Openmode): Add unused attributes.
            * testsuite/27_io/ios_base/types/openmode/case_label.cc: Handle
            noreplace.

Diff:
---
 libstdc++-v3/include/bits/ios_base.h                  | 19 ++++++++++++++-----
 .../27_io/ios_base/types/openmode/case_label.cc       |  4 ++++
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/bits/ios_base.h b/libstdc++-v3/include/bits/ios_base.h
index b69151c49ad..6e343658096 100644
--- a/libstdc++-v3/include/bits/ios_base.h
+++ b/libstdc++-v3/include/bits/ios_base.h
@@ -114,8 +114,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   operator^=(_Ios_Fmtflags& __a, _Ios_Fmtflags __b) _GLIBCXX_NOTHROW
   { return __a = __a ^ __b; }
 
+  // If std::ios_base::noreplace isn't available, -Wswitch should ignore
+  // _S_noreplace.
+#ifdef __glibcxx_ios_noreplace
+#define _NOREPLACE_UNUSED
+#else
+#define _NOREPLACE_UNUSED __attribute__((__unused__))
+#endif
 
-  enum _Ios_Openmode 
+  enum __attribute__((__flag_enum__)) _Ios_Openmode
     { 
       _S_app 		= 1L << 0,
       _S_ate 		= 1L << 1,
@@ -123,12 +130,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _S_in 		= 1L << 3,
       _S_out 		= 1L << 4,
       _S_trunc 		= 1L << 5,
-      _S_noreplace 	= 1L << 6,
-      _S_ios_openmode_end = 1L << 16,
-      _S_ios_openmode_max = __INT_MAX__,
-      _S_ios_openmode_min = ~__INT_MAX__
+      _S_noreplace _NOREPLACE_UNUSED = 1L << 6,
+      _S_ios_openmode_end __attribute__((__unused__)) = 1L << 16,
+      _S_ios_openmode_max __attribute__((__unused__)) = __INT_MAX__,
+      _S_ios_openmode_min __attribute__((__unused__)) = ~__INT_MAX__
     };
 
+#undef _NOREPLACE_UNUSED
+
   _GLIBCXX_NODISCARD _GLIBCXX_CONSTEXPR
   inline _Ios_Openmode
   operator&(_Ios_Openmode __a, _Ios_Openmode __b) _GLIBCXX_NOTHROW
diff --git a/libstdc++-v3/testsuite/27_io/ios_base/types/openmode/case_label.cc b/libstdc++-v3/testsuite/27_io/ios_base/types/openmode/case_label.cc
index 95c86588a54..9e388a51294 100644
--- a/libstdc++-v3/testsuite/27_io/ios_base/types/openmode/case_label.cc
+++ b/libstdc++-v3/testsuite/27_io/ios_base/types/openmode/case_label.cc
@@ -44,6 +44,10 @@ case_labels(bitmask_type b)
       break;
     case std::ios_base::trunc:
       break;
+#ifdef __glibcxx_ios_noreplace
+    case std::ios_base::noreplace:
+      break;
+#endif
     case std::_S_ios_openmode_end:
       break;
     case __INT_MAX__: