std::experimental::propagate_const::operator=
From cppreference.com
                    
                                        
                    < cpp | experimental | propagate const
                    
                                                            
                    |   constexpr propagate_const& operator=( propagate_const&& p ) = default; 
 | 
(1) | (library fundamentals TS v2) | 
|   template<class U> 
constexpr propagate_const& operator=( propagate_const<U>&& pu );  | 
(2) | (library fundamentals TS v2) | 
|   template<class U> 
constexpr propagate_const& operator=( U&& u );  | 
(3) | (library fundamentals TS v2) | 
|   propagate_const& operator=( const propagate_const& ) = delete; 
 | 
(4) | (library fundamentals TS v2) | 
Let t_ designate the private data member that is the wrapped pointer-like object.
1) Explicitly defaulted move assignment operator that move assigns this->t_ from p.t_.
2) Assigns std::move(pu.t_) to this->t_.
This function does not participate in overload resolution unless U is implicitly convertible to T.
3) Assigns std::forward<U>(u) to this->t_.
This constructor does not participate in overload resolution unless U is implicitly convertible to T and std::decay_t<U> is not a specialization of propagate_const
4) Copy assignment operator is explicitly deleted.
[edit] Parameters
| p | - |   another propagate_const object to move from
 | 
| pu | - |   another propagate_const object of a different specialization to move from
 | 
| u | - | another object to assign to the contained pointer | 
[edit] Return value
*this.