Error found:
in module Main
at tests/purs/failing/Superclasses5.purs:17:1 - 18:18 (line 17, column 1 - line 18, column 18)

  A case expression could not be determined to cover all inputs.
  The following additional cases are required to cover all inputs:

    _

  Alternatively, add a Partial constraint to the type of the enclosing value.

while applying a function $__unused
  of type Partial => t0 -> t0
  to argument case $0 of       
                [ x ] -> [ su x
                         ]     
while inferring the type of $__unused (case $0 of      
                                         [ x ] -> [ ...
                                                  ]    
                                      )                
in value declaration suArray

where t0 is an unknown type

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