GHC 7.8.3 here.

Hmm, on second inspection I believe that the Bool reference is to the top-level expression, not the list items after all.

I don't seem to get any defaulting behaviour either when not using GHCi. This seems to indicate that the extended interactive type defaulting is coming into play:

https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_guide/interactive-evaluation.html

I'd guess that this means that the () default is being used. -Wall seems to indicate that this is in-fact the case:

[Prelude] λ [] == []

<interactive>:2:4: Warning:

    Defaulting the following constraint(s) to type ‘()’

      (Eq t0) arising from a use of ‘==’

    In the expression: [] == []

    In an equation for ‘it’: it = [] == []

True

Hope this helps.