Error found:
in module FoldableInstance4
at tests/purs/failing/FoldableInstance4.purs:8:1 - 8:27 (line 8, column 1 - line 8, column 27)

  One or more type variables are in positions that prevent Foldable from being derived.
  To derive this class, make sure that these variables are only used as the final arguments to type constructors, and that those type constructors themselves have instances of Data.Foldable.Foldable or Data.Bifoldable.Bifoldable.

  tests/purs/failing/FoldableInstance4.purs:
      6 
      7 data T a = T (forall t. Show t => t -> a)
      8 derive instance Foldable T


See https://github.com/purescript/documentation/blob/master/errors/CannotDeriveInvalidConstructorArg.md for more information,
or to contribute content related to this error.