[GHC] #8634: Relax functional dependency coherence check ("liberal coverage condition")

[GHC]

#8634: Relax functional dependency coherence check ("liberal coverage condition")
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              Reporter:  danilo2     |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:  7.10.1
              Priority:  high        |           Version:  7.7
             Component:  Compiler    |          Keywords:
            Resolution:              |  Operating System:  Unknown/Multiple
Differential Revisions:  Phab:D69    |   Type of failure:  None/Unknown
          Architecture:              |         Test Case:
  Unknown/Multiple                   |          Blocking:
            Difficulty:  Unknown     |
            Blocked By:              |
       Related Tickets:  #1241,      |
  #2247, #8356, #9103, #9227         |
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Comment (by rwbarton):

 You can write your program with `AllowAmbiguousTypes`: add `{-# LANGUAGE
 AllowAmbiguousTypes, ScopedTypeVariables #-}` and change `tst` to
 {{{
 tst :: forall a s b. (Property a s, Property s b) => a -> b
 tst a = property (property a :: s)
 }}}

 Your program could be equally ambiguous if it used GHC 7.6-style/dys-
 functional dependencies. The difference is that GHC 7.6 assumes that in
 `Property a s`, `a` determines `s` if and only if there is a fundep `a ->
 s`, though neither implication is necessarily true.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:37>
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