[GHC] #8503: New GeneralizedNewtypeDeriving check still isn't permissive enough

[GHC]

#8503: New GeneralizedNewtypeDeriving check still isn't permissive enough
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        Reporter:  goldfire          |            Owner:  goldfire
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #8541
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Comment (by goldfire):

 Yes, the depth-bound seems sensible and is probably the only real solution
 to this problem.

 We should be aware of a particularly nasty case, like this:

 {{{
 Coercible (Maybe (Fix Maybe)) (Fix Maybe)
 }}}

 This should be solvable (and I posit that more sensible cases might come
 up in practice). The current algorithm simplifies the left-hand type (LHT)
 as far as it can, then simplifies the right-hand type (RHT) as far as it
 can. I would worry that the depth would hit its maximum when reducing the
 LHT and then refuse to simplify the RHT. Even if we tracked depths
 separately for the two sides, the right side would hit the maximum depth
 right before the two sides unify, leading to failure, no matter what the
 depth is!

 Note that if we just switch the order of the arguments to `Coercible`,
 this goal is easy to solve, after one reduction on the LHT. Having the
 solubility of a symmetric constraint depend on the order of arguments is
 undesirable.

 I have no good ideas of how to fix this, at the moment.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8503#comment:21>
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