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I just updated the GHC 8.0 snapshot I was using from 8.0.0.20160214-g977fb8 to 8.0.0.20160218-g23baff79, and suddenly hackage:microlens-0.4.2.0 fails to build with

...
Preprocessing library microlens-0.4.2.0...
[1 of 4] Compiling Lens.Micro.Type  ( src/Lens/Micro/Type.hs, dist/build/Lens/Micro/Type.o )
[2 of 4] Compiling Lens.Micro.Internal ( src/Lens/Micro/Internal.hs, dist/build/Lens/Micro/Internal.o )

src/Lens/Micro/Internal.hs:184:3: error:
    • Couldn't match type ‘s’ with ‘g0 a’
        ‘s’ is untouchable
          inside the constraints: (Traversable g,
                                   s ~ g a,
                                   t ~ g b,
                                   Applicative f)
          bound by the type signature for:
                     each :: (Traversable g, s ~ g a, t ~ g b, Applicative f) =>
                             (a -> f b) -> s -> f t
          at src/Lens/Micro/Internal.hs:184:3-27
    • In the ambiguity check for ‘each’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method:
        each :: forall s t a b. Each s t a b => Traversal s t a b
      In the class declaration for ‘Each’

1. ..is this a regression or a feature? :-)

Trac metadata

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