Converting std::unique_ptr<Derived> to std::unique_ptr<Base>

The compiler's behaviour is correct. There is only an implicit move when the types are the same, because implicit move is specified in terms of the compiler failing to perform copy elision in cases where it is actually allowed (see 12.8/31 and 12.8/32).

12.8/31 (copy elision):

in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv-unqualified type as the function return type...

12.8/32 (implicit move):

When the criteria for elision of a copy operation are met, [...], overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue.