[RFC] libstdc++: optional: Set _M_engaged after the call to destructor [PR127005]

[Andrea Pinski]

Sometimes the destructor will cause the optimizers not to see that
_M_engaged has been to set to false (e.g. an external call). This
means that we might get an uninitialized warning after the destructor
in some cases.
Setting it after the destructor will cause one extra store in some cases
but that store allows the optimizers to get rid of other code.

This is a RFC because I don't know if we want to remove the first store
of _M_engaged or keep it.
The patch has been bootstrapped and tested on x86_64-linux-gnu with no regressions.
And tested provided in the bug report works and also the generated code looks much
better too.

	PR libstdc++/127005

libstdc++-v3/ChangeLog:

	* include/std/optional (optional::_M_destroy): Set _M_engaged
	to false after the destructor call.

Signed-off-by: Andrea Pinski <andrew.pinski@oss.qualcomm.com>
---
 libstdc++-v3/include/std/optional | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index d6fd33e8c2e..8884c54f3e7 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -303,6 +303,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	_M_engaged = false;
 	_M_payload._M_value.~_Stored_type();
+	_M_engaged = false;
 #if defined(__clang__) && __cpp_lib_optional >= 202106L // full constexpr support
 	if (std::is_constant_evaluated())
 	  // Work around PR124910 for Clang.
-- 
2.43.0