[GHC] #11264: Previously compiling example does not compile

[GHC]

#11264: Previously compiling example does not compile
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  typecheck/should_compile/ClassOperator
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * testcase:   => typecheck/should_compile/ClassOperator
 * failure:  None/Unknown => GHC rejects valid program
 * milestone:   => 8.0.1
 * priority:  normal => high


Old description:



New description:

 The example in the testcase introduced in Phab:D1667 currently fails with,
 {{{
 ClassOperator.hs:12:3: error:
     • Could not deduce (a ><> b0)
       from the context: a ><> b
         bound by the type signature for:
                    (**>) :: (a ><> b) => a -> a -> ()
         at ClassOperator.hs:12:3-44
       The type variable ‘b0’ is ambiguous
     • In the ambiguity check for ‘**>’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       When checking the class method:
         (**>) :: forall a b. (a ><> b) => a -> a -> ()
       In the class declaration for ‘><>’

 ClassOperator.hs:12:3: error:
     • Could not deduce (a ><> b0)
       from the context: a ><> b
         bound by the type signature for:
                    (**<) :: (a ><> b) => a -> a -> ()
         at ClassOperator.hs:12:3-44
       The type variable ‘b0’ is ambiguous
     • In the ambiguity check for ‘**<’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       When checking the class method:
         (**<) :: forall a b. (a ><> b) => a -> a -> ()
       In the class declaration for ‘><>’

 ClassOperator.hs:12:3: error:
     • Could not deduce (a ><> b0)
       from the context: a ><> b
         bound by the type signature for:
                    (>**) :: (a ><> b) => a -> a -> ()
         at ClassOperator.hs:12:3-44
       The type variable ‘b0’ is ambiguous
     • In the ambiguity check for ‘>**’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       When checking the class method:
         (>**) :: forall a b. (a ><> b) => a -> a -> ()
       In the class declaration for ‘><>’

 ClassOperator.hs:12:3: error:
     • Could not deduce (a ><> b0)
       from the context: a ><> b
         bound by the type signature for:
                    (<**) :: (a ><> b) => a -> a -> ()
         at ClassOperator.hs:12:3-44
       The type variable ‘b0’ is ambiguous
     • In the ambiguity check for ‘<**’
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       When checking the class method:
         (<**) :: forall a b. (a ><> b) => a -> a -> ()
       In the class declaration for ‘><>’
 }}}

 This is a regression relative to 7.10.2, which accepts the program without
 error.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11264#comment:1>
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