R operator()(ArgTypes... args) const

1 Effects: INVOKE (f, std::forward<ArgTypes>(args)..., R) (20.9.2), where f is the target object
(20.9.1) of *this.
2 Returns: Nothing if R is void, otherwise the return value of INVOKE (f, std::forward<ArgTypes>(
args)..., R).

_>std::function<void(void)> f = [](void) -> bool {return true;};
_>

[func.require]
Define INVOKE (f, t1, t2, ..., tN, R) as INVOKE (f, t1, t2, ..., tN) implicitly converted to R.

std::function<void(void)> f = [](void) -> bool {return true;};

Any expression can be explicitly converted to type cv void

template<class F> function(F f);
7 Requires: F shall be CopyConstructible. f shall be Callable (20.9.11.2) for argument types ArgTypes
and return type R. The copy constructor and destructor of A shall not throw exceptions.

A callable object f of type F is Callable for argument types ArgTypes and return type R if the expression
INVOKE(f, declval<ArgTypes>()..., R), considered as an unevaluated operand (Clause 5), is well
formed (20.9.2).

Define INVOKE (f, t1, t2, ..., tN, R) as INVOKE (f, t1, t2, ..., tN) implicitly converted to R.