[PATCH] libstdc++: Restrict engaged -> disengaged std::optional fix [PR124910]

[Patrick Palka]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

-- >8 --

It turns out a union without an active member does not violate C++20 core
constant expression rules and r16-8748 was really just a workaround for
a front end bug.  The actual underlying problem -- that the constexpr
evaluator treated an explicitly destroyed union member as still active
-- has been fixed by r16-8767 which makes this workaround unnecessary
for GCC.  Rather than remove the workaround, restrict it to Clang which
seems to have a similar bug.

	PR c++/124910

libstdc++-v3/ChangeLog:

	* include/std/optional (_Optional_payload_base::_M_destroy):
	Restrict r16-8748 workaround to Clang, and adjust comment.
---
 libstdc++-v3/include/std/optional | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index 49ba7b6b45dd..b9cc019feff7 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -321,10 +321,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	_M_engaged = false;
 	_M_payload._M_value.~_Stored_type();
-#if __cpp_lib_optional >= 202106L // full constexpr support
+#if defined(__clang__) && __cpp_lib_optional >= 202106L // full constexpr support
 	if (std::is_constant_evaluated())
-	  // Ensure union _M_payload always has an active member, for sake
-	  // of the core constant expression requirements.
+	  // Work around PR124910 for Clang.
 	  std::construct_at(std::__addressof(_M_payload._M_empty));
 #endif
       }
-- 
2.54.0.rc1.54.g60f07c4f5c