future: avoid inheriting from future payload type #2598
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The uninitialized_wrapper_base will, when possible inherit from the future payload type T in order to apply the empty base optimization (if sizeof(T) == 0). However, this injects methods from T into the scope of uninitialized_wrapper and classes that inherit from it, such as future_state. If one of the names of the methods in future_state clashes with a member of T, we get a compilation failure. Fix by avoiding the inheritance branch of uninitialized_wrapper_base and using the composition branch, and use [[no_unique_address]] to avoid bloating future<some_empty_class>. It turns out that we delete the specialization that was used with internal::monostate (corresponding to future<>), and so we have to specialize uninitialized_wrapper for it. I verified that sizeof(future<>) isn't changed by the patch.
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The uninitialized_wrapper_base will, when possible inherit from the future payload type T in order to apply the empty base optimization (if sizeof(T) == 0). However, this injects methods from T into the scope of uninitialized_wrapper and classes that inherit from it, such as future_state. If one of the names of the methods in future_state clashes with a member of T, we get a compilation failure.
Fix by avoiding the inheritance branch of uninitialized_wrapper_base and using the composition branch, and use [[no_unique_address]] to avoid bloating future<some_empty_class>.
It turns out that we delete the specialization that was used with internal::monostate (corresponding to future<>), and so we have to specialize uninitialized_wrapper for it.
I verified that sizeof(future<>) isn't changed by the patch.