λ. :set -XFlexibleContexts -XFlexibleInstances -XMultiParamTypeClasses -XUndecidableInstances

λ. import RIO

λ. import Data.Has

λ. class HasBool a where boolL :: Lens' a Bool

λ. instance {-#OVERLAPPABLE#-} HasBool a => Has Bool a where hasLens = boolL

λ. instance {-#OVERLAPPABLE#-} Has Bool a => HasBool a where boolL = hasLens

λ. runRIO () $ view boolL

^CInterrupted.

The RIO environment () doesn't contain a Bool. I can see how the typechecker might get lost in this, but I can compile an equivalent program; it loops on execution.

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