[GHC] #10715: Possible regression in Coercible a (X a) between 7.8 and 7.10

[GHC]

#10715: Possible regression in Coercible a (X a) between 7.8 and 7.10
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        Reporter:  inaki             |                   Owner:  goldfire
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by goldfire):

 Replying to [comment:11 nomeata]:
 > Richard , would it be unreasonable to support recursive newtypes where
 the occurrences of the newtype are all in phantom positions?

 In a word: maybe. Though I haven't actually sat down to prove it, I very
 strongly suspect that solving for `Coercible` is undecidable in the
 presence of recursive newtypes. In effect, we are solving for
 equirecursive type equality. This can be done, but the algorithm I have to
 hand (from Pierce's TAPL, Chapter 21) requires (translating to the
 language of `Coercible`) that there be only one way to decompose an
 equality. But that's not true here! If we have `[W] N a ~R N b`, where `N`
 is a newtype, there are //two// ways forward: unwrap the newtype and try
 again, or look at the type parameters `a` and `b` (according to `N`'s
 parameter's role). So Pierce's algorithm doesn't work out.

 We are left, then, with an incomplete algorithm. I'm confident that we
 could keep adding special cases to this algorithm to cover yet-weirder
 scenarios, but I think it's best to wait until we have a better
 motivation. (This ticket doesn't qualify, because there is a better way to
 do what the author wants.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10715#comment:13>
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