Error found:
in module TypeWildcards
at tests/purs/failing/TypeWildcards3.purs:8:1 - 9:19 (line 8, column 1 - line 9, column 19)

  Type class instance head is invalid due to use of type
     
    _
     
  All types appearing in instance declarations must be of the form T a_1 .. a_n, where each type a_i is of the same form, unless the type is fully determined by other type class arguments via functional dependencies.

in type class instance
                        
  Data.Show.Show (Foo _)
                        

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