small hiccup in new GND check

[Richard Eisenberg]

Hi all,

The implementation of the new GND check is done and was rather straightforward. But, it brings up a new design issue: With the old check, a writer of a class C could say `type role C nominal` to prevent GND use with that class. Now, all classes have nominal parameters, and there is no (clean) way for a class writer to stop the use of GND. Perhaps this is OK -- classes can't in general be abstract -- but I thought we should pause a moment to reflect on this consequence of the design.

For the curious, you can see the code in my github fork at https://raw.github.com/goldfirere/ghc/master/compiler/typecheck/TcDeriv.lhs (search for `role_errs`). An example of the new error message is in https://github.com/goldfirere/testsuite/blob/master/tests/roles/should_fail/Roles10.stderr

I'm running further tests now, so wibbles to the above may happen.

Richard
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