[libc++] `std::function` with small object optimization copies instead of moves

[BenjaminSchaaf]

When using std::function I noticed that sometimes the captured data is copied even if the std::function is only ever moved.

I've tracked this down to the following code:

llvm-project/libcxx/include/__functional/function.h

Line 394 in 6003c30

__f_ = __as_base(&__buf_);

It looks like when the small object optimization is in use for std::function, then the captured data is copied instead of moved whenever the function is moved. In extreme cases this could result in massively more work being done.

The issue reproduces trivially, see godbolt: https://godbolt.org/z/czTev8jfc

Note that the captured data needs to be < 3*sizeof(void *) and have noexcept on its copy constructor to trigger the small object optimization. When that's the case the data is copied instead of moved.

[frederick-vs-ja]

Duplicate of #32472.