[GHC] #14984: Regression: 8.4.1 says "Could not deduce" when before 8.4 it could (was: Rgression: 8.4.1 says "Could not deduce" when before 8.4 it could)

[GHC]

#14984: Regression: 8.4.1 says "Could not deduce" when before 8.4 it could
-------------------------------------+-------------------------------------
        Reporter:  erikd             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.4.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I must be missing something, because this program does not typecheck for
 me on GHC 8.2.2 //or// 8.4.1:

 {{{
 $ /opt/ghc/8.2.2/bin/ghci Bug2.hs
 GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug2.hs, interpreted )

 Bug2.hs:5:18: error:
     • Could not deduce (Testable a0) arising from a use of ‘monadicIO’
       from the context: Testable a
         bound by the type signature for:
                    testPropertyIO :: forall a.
                                      Testable a =>
                                      PropertyM IO a -> Property
         at Bug2.hs:4:1-58
       The type variable ‘a0’ is ambiguous
       These potential instances exist:
         instance [safe] Testable prop => Testable (Gen prop)
           -- Defined in ‘Test.QuickCheck.Property’
         instance [safe] Testable Discard
           -- Defined in ‘Test.QuickCheck.Property’
         instance [safe] Testable Property
           -- Defined in ‘Test.QuickCheck.Property’
         ...plus three others
         ...plus two instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘(.)’, namely ‘monadicIO’
       In the expression: monadicIO . (=<<) stop
       In an equation for ‘testPropertyIO’:
           testPropertyIO = monadicIO . (=<<) stop
   |
 5 | testPropertyIO = monadicIO . (=<<) stop
   |                  ^^^^^^^^^
 Failed, no modules loaded.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14984#comment:1>
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