Summary:
The issue is that std::swap must have the same type for both parameters but the member .swap() for PolyVal takes Poly and then uses std::swap(*this, that).

the fix uses static_cast to change the Poly to a PolyVal for std::swap

Reviewed By: ericniebler

Differential Revision: D7730948

fbshipit-source-id: 8dd93fc3c86b87938a7c0c12ccb3b5209a593730