[CUDA] Fix a misuse of std::move: address of stack memory associated with temporary object of type std::lock_guard<std::mutex> returned to caller

[PragmaTwice]

Related issue = #

In the original code, the returned rvalue reference is obviously a dangling reference, and the scoped mutex has no effect (destructed immediately after construction).

[CLAassistant]

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[k-ye]

LGTM && welcome to the community!

[PragmaTwice]

Some fact to help understand it:

• std::move is implicit and mandatory while a local variable is returned (in the standard, the overload resolution will select a move ctor T(T&&) first, as treat the lvalue as an rvalue, since c++11, addressed in ISO/IEC 14882:2011 12.8/32)
• RVO (but not NRVO) is a mandatory optimization since c++17, so even move ctor will not be called here. Using std::move here will prevent RVO. (of course, the return type can never be a reference type while you want to return a local variable (with auto storage duration))

[ailzhang]

Nice catch! Thank you!