[func.wrap.move.ctor]/22 and [func.wrap.move.ctor]/25 define the effects of assignment as following:

move_only_function& operator=(move_only_function&& f);

Effects: Equivalent to: move_only_function(std::move(f)).swap(*this);

[…]

template<class F> move_only_function& operator=(F&& f);

Effects: Equivalent to: move_only_function(std::forward<F>(f)).swap(*this);

The assignment via swap with temporary makes the implementation to do the following:

• move out the previous target to a temporary location

• move in the new target

• finally destroy the previous target.

As everything is noexcept here, I think it can be short cut to just:

• destroy the previous target.

• move in the new target

Looks like the implementation cannot do such optimization in a generic case with small functor optimization enabled and non-trivial move constructor for the new target and with non-trivial destruction of the previous target, since the difference is observable.

Apparently the optimization is precluded for no reason.