ezyang@sabre:~$ ghc-8.0 --interactive
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Prelude> \x -> \x -> _

<interactive>:1:13: error:
    • Found hole: _ :: t2
      Where: ‘t2’ is a rigid type variable bound by
               the inferred type of it :: t -> t1 -> t2 at <interactive>:1:1
    • In the expression: _
      In the expression: \ x -> _
      In the expression: \ x -> \ x -> _
    • Relevant bindings include
        x :: t1 (bound at <interactive>:1:8)
        x :: t (bound at <interactive>:1:2)
        it :: t -> t1 -> t2 (bound at <interactive>:1:1)

GHC should not report bindings which are shadowed.