[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):

 Adding:
 {{{
 {-# LANGUAGE NoMonomorphismRestriction #-}
 {-# LANGUAGE FlexibleContexts #-}
 }}}
 produces a different message:
 {{{
 InjectiveInference.hs:36:8: error:
     • Could not deduce: ProcessOf t0 ~ ProcessOf t
       from the context: CausalProcess (ProcessOf t)
         bound by the inferred type for ‘osci’:
                    CausalProcess (ProcessOf t) => ProcessOf t Float y
         at InjectiveInference.hs:36:8-26
       Expected type: ProcessOf t Float y
         Actual type: ProcessOf t0 Float y
       NB: ‘ProcessOf’ is a type function, and may not be injective
       The type variable ‘t0’ is ambiguous
     • In the ambiguity check for the inferred type for ‘osci’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       When checking the inferred type
         osci :: forall y (t :: * -> *).
                 CausalProcess (ProcessOf t) =>
                 ProcessOf t Float y
       In the expression: let osci = shapeModOsci in osci $& frequencies
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12526#comment:8>
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