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

[GHC]

#12526: regression in type inference with respect to type families
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        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:                    |
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Comment (by niteria):

 This reproduces in HEAD (from Aug 9) with:

 {{{
 InjectiveInference.hs:35:8: error:
     • Couldn't match type ‘ProcessOf t0’ with ‘Causal’
       Expected type: Causal Float Float
         Actual type: ProcessOf t0 Float Float
       The type variable ‘t0’ is ambiguous
     • In the first argument of ‘($&)’, namely ‘osci’
       In the expression: osci $& frequencies
       In the expression: let osci = shapeModOsci in osci $& frequencies
     • Relevant bindings include
         osci :: forall y. ProcessOf t0 Float y
           (bound at InjectiveInference.hs:34:8)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12526#comment:7>
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