[GHC] #9443: Regression from 7.6.3 to 7.8.3: could not coerce because argument "has role Nominal"

[GHC]

#9443: Regression from 7.6.3 to 7.8.3: could not coerce because argument "has role
Nominal"
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              Reporter:  bernalex    |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  x86_64 (amd64)
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  GHC         |       Blocked By:
  rejects valid program              |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 I don't think this is a bug, but I suppose I could be convinced otherwise.

 Consider this:

 {{{#!hs
 class C a where
   meth :: D a => a -> a
 }}}

 Note that the constraint on `meth` is ''not'' `C a`, but some other class
 `D a`. If we wish to derive an instance of `C` via GND, the derived
 instance will use the exact same implementation of `meth` as does the
 original instance. BUT, the newtype (say, `Year a`) may have a
 ''different'' instance for `D` compared to the representation type
 (`Integer`). So, reusing the old implementation would be bogus. That's why
 class parameters default to have nominal roles.

 Of course, if we knew that the `D` instances were in fact the same, then
 the derivation makes good sense. But GHC doesn't track "instance equality"
 (which could just mean that one instance is the GND of the other). In the
 case presented here, the instance equality is guaranteed, as we're
 performing the GND to make the instances equal. However, detecting this,
 in general, is hard, and this seems like enough of a corner case to let it
 go unfixed.

 Does that explain it?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9443#comment:4>
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