[GHC] #8565: New GeneralisedNewtypeDeriving needs help with higher rank types

[GHC]

#8565: New GeneralisedNewtypeDeriving needs help with higher rank types
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        Reporter:  simonpj                       |            Owner:
            Type:  bug                           |           Status:  new
        Priority:  normal                        |        Milestone:
       Component:  Compiler                      |          Version:  7.6.3
      Resolution:                                |         Keywords:
Operating System:  Unknown/Multiple              |     Architecture:
 Type of failure:  None/Unknown                  |  Unknown/Multiple
       Test Case:                                |       Difficulty:
  typecheck/should_compile/T8565                 |  Unknown
        Blocking:                                |       Blocked By:
                                                 |  Related Tickets:
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Comment (by simonpj):

 It should be fine.  Solving `Coercible s t` constraints is very very like
 solving `s ~ t` constraints.  Just as we have a rule for `[s] ~ [t]` (by
 solving the constraint `s~t`), so we have a rule for `Coercible [s] [t]`.

 Now, we do have a rule for `(forall a. s) ~ (forall a. t)`, but it's a bit
 tricky because of the scoping of `a`.  We have to solve the implication
 constraint `forall a. s~t`.  To see how we do this for equality
 constraint, look on line 745 of `TcCanonical`, and the key support
 function `TcSMonad.deferTcSForAllEq`.  The latter is the whole reason for
 `TcLetCo`, which was a significant innovation at the time. (Previously we
 could not decompose `forall/forall` equalities.)  But now it's there, you
 should be able to use the exact same strategy for `Coercible`.  (Provided
 you complete the `EvCoercible` refactoring you started.)

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8565#comment:9>
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