It's tricky to set up the context for a Data instance.  I got it wrong
once, and fixed it -- hence the "extra_constraints" in
TcDeriv.inferConstraints.  

But it still wasn't right!  The tricky bit is that dataCast1 is only
generated when T :: *->*, and dataCast2 when T :: *->*->*. (See
the code in TcGenDeriv for dataCastX.