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[libcxx] LWG4172 fix self-move-assignment in {unique|shared}_lock #129542

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[libcxx] LWG4172 fix self-move-assignment in {unique|shared}_lock #129542

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8 changes: 1 addition & 7 deletions libcxx/include/__mutex/unique_lock.h
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Original file line number Diff line number Diff line change
Expand Up @@ -74,13 +74,7 @@ class unique_lock {
}

_LIBCPP_HIDE_FROM_ABI unique_lock& operator=(unique_lock&& __u) _NOEXCEPT {
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@mordante mordante Mar 3, 2025

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Typically for this LWG issue you need to update the synopsis due to the adding of noexcept, but that seems already done.

if (__owns_)
__m_->unlock();

__m_ = __u.__m_;
__owns_ = __u.__owns_;
__u.__m_ = nullptr;
__u.__owns_ = false;
unique_lock{std::move(__u)}.swap(*this);
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@frederick-vs-ja frederick-vs-ja Oct 24, 2025

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You should include <__utility/move.h> in <__mutex/unique_lock.h> and <shared_mutex>. And then you need to add

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

immediately before

_LIBCPP_BEGIN_NAMESPACE_STD

, and

_LIBCPP_POP_MACROS

immediately after

_LIBCPP_END_NAMESPACE_STD

return *this;
}

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9 changes: 1 addition & 8 deletions libcxx/include/shared_mutex
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Expand Up @@ -340,14 +340,7 @@ public:
}

_LIBCPP_HIDE_FROM_ABI shared_lock& operator=(shared_lock&& __u) _NOEXCEPT {
if (__owns_)
__m_->unlock_shared();
__m_ = nullptr;
__owns_ = false;
__m_ = __u.__m_;
__owns_ = __u.__owns_;
__u.__m_ = nullptr;
__u.__owns_ = false;
shared_lock{std::move(__u)}.swap(*this);
return *this;
}

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23 changes: 16 additions & 7 deletions .../thread.mutex/thread.lock/thread.lock.shared/thread.lock.shared.cons/move_assign.pass.cpp
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Expand Up @@ -21,22 +21,31 @@

#include "test_macros.h"


int main(int, char**)
{
{
int main(int, char**) {
{
typedef std::shared_timed_mutex M;
M m0;
M m1;
std::shared_lock<M> lk0(m0);
std::shared_lock<M> lk1(m1);

// Test self move assignment for lk0.
lk0 = std::move(lk0);
assert(lk0.mutex() == std::addressof(m0));
assert(lk0.owns_lock() == true);

lk1 = std::move(lk0);
assert(lk1.mutex() == std::addressof(m0));
assert(lk1.owns_lock() == true);
assert(lk0.mutex() == nullptr);
assert(lk0.owns_lock() == false);
}
{

// Test self move assignment for lk1.
lk1 = std::move(lk1);
assert(lk1.mutex() == std::addressof(m0));
assert(lk1.owns_lock() == true);
}
{
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@mordante mordante Mar 3, 2025

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I noticed this test does not validate the noexcept status of the function, can you add that.
(The next test should add it since there it's a new requirement.)
The test for swap has an example how to test for noexcept.

typedef nasty_mutex M;
M m0;
M m1;
Expand All @@ -47,7 +56,7 @@ int main(int, char**)
assert(lk1.owns_lock() == true);
assert(lk0.mutex() == nullptr);
assert(lk0.owns_lock() == false);
}
}

return 0;
}
11 changes: 10 additions & 1 deletion .../thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/move_assign.pass.cpp
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Expand Up @@ -24,13 +24,22 @@ int main(int, char**) {
std::unique_lock<checking_mutex> lk0(m0);
std::unique_lock<checking_mutex> lk1(m1);

// Test self move assignment for lk0.
lk0 = std::move(lk0);
assert(lk0.mutex() == std::addressof(m0));
assert(lk0.owns_lock() == true);

auto& result = (lk1 = std::move(lk0));

assert(&result == &lk1);
assert(lk1.mutex() == std::addressof(m0));
assert(lk1.owns_lock());
assert(lk0.mutex() == nullptr);
assert(!lk0.owns_lock());
assert(lk0.owns_lock() == false);

// Test self move assignment for lk1
lk1 = std::move(lk1);
assert(lk1.mutex() == std::addressof(m0));
assert(lk1.owns_lock() == true);
return 0;
}
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