[GHC] #12526: regression in type inference with respect to type families

[GHC]

#12526: regression in type inference with respect to type families
-------------------------------------+-------------------------------------
        Reporter:  Lemming           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.2
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  10634             |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by Lemming):

 * related:   => 10634


Comment:

 Replying to [comment:4 niteria]:
 > > It seems that this might be related to the work that Bartosz has done
 in making GHC more deterministic.
 >
 > > That is, the error already existed in ghc-8.0.1 but was not always
 triggered.
 >
 > Most of the determinism patches were merged *after* ghc-8.0.1, so it
 sounds unlikely.

 Why? GHC-8.0.1 showed the error sometimes, GHC-8.0.2 shows it always.
 mpickering's guess matches perfectly.

 > Lemming: Can you provide step by step repro?

 I am currently reducing the example. It looks like another instance of the
 fake injective type functions problem.
 https://ghc.haskell.org/trac/ghc/ticket/10634

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12526#comment:5>
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