In #4025 there's attemption to strengthen the exception specification of _Ref_fn::operator().

Given the standard class template std::reference_wrapper is already a invocable wrapper and its operator() has a propagating exception specification (thanks to LWG-3764), it seems OK to replace the uses of _Ref_fn with reference_wrapper.

Pros:

• user codes should only use reference_wrapper, so we might be able to instantiate less specializations.

Cons:

• reference_wrapper is slightly heavier than _Ref_fn;
• reference_wrapper is assignable, which might be unwanted for the internal details of the implementation.