[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:  patch
        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:  Phab:D1263
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Comment (by Richard Eisenberg <eir@…>):

 In [changeset:"2f9809efdbc11fee445dbe3d5c555433ec3c5e6a/ghc"
 2f9809ef/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2f9809efdbc11fee445dbe3d5c555433ec3c5e6a"
 Slightly better `Coercible` errors.

 This makes two real changes:
  - Equalities like (a ~R [a]) really *are* insoluble. Previously,
    GHC refused to give up when an occurs check bit on a representational
    equality. But for datatypes, it really should bail.

  - Now, GHC will sometimes report an occurs check error (in cases above)
    for representational equalities. Previously, it never did.

 This "fixes" #10715, where by "fix", I mean clarifies the error message.
 It's unclear how to do more to fix that ticket.

 Test cases: typecheck/should_fail/T10715{,b}
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10715#comment:15>
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