Constexpr-ness of std :: optional <T> :: value_or

Question

As shown on this page , the version of ref std::optional<T>::value_or not defined by constexpr :

 template< class U > constexpr T value_or( U&& default_value ) const&; template< class U > T value_or( U&& default_value ) &&; 

What are the reasons for this? Is this because moving a value can change an optional object?

But if we make the && version also constexpr , it is not good that we can now write:

 constexpr int x = std::optional<int>(123).value_or(456);