TcValidity.fvTypes works in partnership with sizeTypes, and
hence should ignore kinds in exactly the same way.  It wasn't
doing so, which meant that validDerivPred said "No" when it
should have said "Yes". That led to the bug reported in
Trac #10524 comment:7.

The error message is pretty terrible
  No instance for (Typeable T)
but I'll fix that next