[ispeters]

__apply and __get pass on each element of a forwarded __tuple by naming it as a member of the forwarded tuple, but naming a member of reference type gives an lvalue whatever the tuple's value category. So an element declared as T && is passed on from a moved tuple as T &, although __apply's constraint, noexcept and return type describe it as T &&, and __get already returns it as T && from a tuple of more than eight elements. A function that can take only an rvalue can't be applied, and one that takes the element by value copies it instead of moving it.

exec::function stores its curried arguments in a __tuple<_Args...>, so a function whose call type takes a curried argument of rvalue reference type, such as function<std::string(std::string &&)>, can be constructed but not connected: connect applies to the moved tuple a function that takes the argument as std::string &&.

This diff casts each element to its declared type with the forwarded tuple's cvref-qualifiers, as std::get does and as __structured_apply already does, in __apply for tuples of every size and in __get for tuples of up to eight elements. Elements of value or lvalue reference type are passed on as before.

The new tests check what __apply and __get give for elements of value, lvalue reference and rvalue reference type, from small and large tuples, and that a function passes its curried arguments to the factory as their declared types.

[copy-pr-bot]

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