While implementing P2505R5 "Monadic Functions for std::expected" we found it odd that the template type parameter for the assignment operator that accepts an argument by forwarding reference is defaulted, but the template type parameter for value_or is not. For consistency, it would seem that meow.value_or(woof) should accept the same arguments woof as does meow = woof, even when those arguments are braced-initializers.

That said, it would be peculiar to default the template type parameter of value_or to T instead of remove_cv_t<T>. For expected<const vector<int>, int> meow{unexpect, 42};, for example, meow.value_or({1, 2, 3}) would create a temporary const vector<int> for the argument and return a copy of that argument. Were the default template argument instead remove_cv_t<T>, meow.value_or({1, 2, 3}) could move construct its return value from the argument vector<int>. For the same reason, the constructor that accepts a forwarding reference with a default template argument of T should default that argument to remove_cv_t<T>.

For consistency, it would be best to default the template argument of the perfect-forwarding construct, perfect-forwarding assignment operator, and value_or to remove_cv_t<T>. Since all of the arguments presented apply equally to optional, we believe optional should be changed consistently with expected. MSVCSTL has prototyped these changes successfully.