[GHC] #10991: Don't permit type variables in the context unless they are in the type

[GHC]

#10991: Don't permit type variables in the context unless they are in the type
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        Reporter:  bernalex          |                Owner:  bernalex
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by thomie):

 Which version of GHC are you using that this is accepted? ghc-7.10.2
 reports:

 {{{
 Test.hs:3:6:
     Could not deduce (Eq a0)
     from the context (Eq a)
       bound by the type signature for f :: Eq a => b -> b
       at Test.hs:3:6-19
     The type variable ‘a0’ is ambiguous
     In the ambiguity check for the type signature for ‘f’:
       f :: forall b a. Eq a => b -> b
     To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
     In the type signature for ‘f’: f :: Eq a => b -> b
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10991#comment:1>
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