This is set out in the C++11 standard, § 12.8/32:
When the criteria for elision of a copy operation are met or would be
met save for the fact that the source object is a function parameter,
and the object to be copied is designated by an lvalue, overload
resolution to select the constructor for the copy is first performed
as if the object were designated by an rvalue....
(emphasis mine). In plain english, it means that the lvalue p can be treated as an rvalue when it comes to overload resolution, because it is a candidate for copy elision. This in turn means the move constructor is picked up on overload resolution (in actual fact, the move copy is probably elided anyway.)
