Regression: 8.4.1 says "Could not deduce" when before 8.4 it could

Have some code:

import           Test.QuickCheck
import           Test.QuickCheck.Monadic

testPropertyIO :: Testable a => PropertyM IO a -> Property
testPropertyIO = monadicIO . (=<<) stop

That compiles fine with ghc 7.10 through 8.2, but with 8.4.1 fails with:

test/Test/Mafia/IO.hs:18:18: 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 test/Test/Mafia/IO.hs:17:1-58
      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

Either the compiler has been incorrectly accepting this as valid for a number of releases or this is a regression.

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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