Describe the bug
According to [structure.specifications]/4, some uses of "Effects: Equivalent to" specifies "Constraints:" of the underlying function calls. I suppose that it only propagates "Constraints:" specified for standard library functions, not literal requires or other substitution failures, and the implied "Constraints:" were specified by "Remarks:" in C++11~17 (in other words, essentially unchanged in C++20).

From a quick glance at WG21-N4901, I think the following functions have additional SFINAE constraints (and currently there're some defects in N4901):

• template overloads of std::shared_ptr::operator=
• template overloads of std::shared_ptr::reset
• template overloads of std::weak_ptr::operator=
• std::make_optional
• std::make_any
• the template overload of std::move_only_function::operator=
  (Anything missing?)

Currently none of these functions is constrained in MSVC STL (except for std::move_only_function::operator=, which is being implemented).

As all of these functions have the same "Constraints:" as the selected constructors, perhaps we can simply add enable_if_t<is_constructible_v<...>, int> = 0 to them.

I have submitted 2 related LWG issues:

• LWG-3627: std::make_optional overloads are inconsitently specified. IMO they should all be constrained.
• LWG-3628: uninitialized algorithms and some std::reduce overloads are also specified by "Effects: Equivalent to", but I feel that they are misspecified and should not have additional SFINAE constraints.