[GHC] #10009: type inference regression when faking injective type families

[GHC]

#10009: type inference regression when faking injective type families
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        Reporter:  aavogt            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:  7.12.1
       Component:  Compiler (Type    |                 Version:  7.10.1-rc1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:  #10226            |
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Comment (by edsko):

 Reported this independently, didn't see this ticket, sorry. But we were
 just bitten by this in ide-backend as well (we can work around it with
 proxies). See https://ghc.haskell.org/trac/ghc/ticket/10226 for an
 independent example, might perhaps be useful as a second test case (and
 some detailed comments there as well). Note that I do *not* think, as per
 @simonpj 's comment above, that "It can always be fixed by adding a type
 signature".

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10009#comment:17>
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