Cancellation slot requirements

A type X meets the CancellationSlot requirements if it satisfies the requirements of CopyConstructible (C++Std [copyconstructible]) and Destructible (C++Std [destructible]), as well as the additional requirements listed below.

In the table below, x1 and x2 denote (possibly const) values of type X, mx1 denotes an xvalue of type X, H denotes a type that satisfies CancellationHandler, h denotes an xvalue of type H, and u0 to uN denote identifiers.

Table 8. CancellationSlot requirements

expression	type	assertion/note pre/post-conditions
`X u(x1);`		Shall not exit via an exception. post: `u == x1`.
`X u(mx1);`		Shall not exit via an exception. post: `u` equals the prior value of `mx1`.
`x1 == x2`	`bool`	Returns `true` only if `x1` and `x2` can be interchanged with identical effects. `operator==` shall be reflexive, symmetric, and transitive, and shall not exit via an exception.
`x1 != x2`	`bool`	Same as `!(x1 == x2)`.
`x1.assign(h)`	`H&`
`x1.emplace<H>(u0, ..., uN)`	`H&`	Requires that `is_constructible<H, decltype(u0), ..., decltype(u0)>::value` be true.