Error found:
in module X
at tests/purs/failing/2806.purs:6:1 - 6:29 (line 6, column 1 - line 6, column 29)

  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 => t1 -> t1
  to argument case e of          
                e | L x <- e -> x
while checking that expression $__unused (case e of          
                                            e | L x <- e -> x
                                         )                   
  has type a0
in value declaration g

where a0 is a rigid type variable
        bound at (line 0, column 0 - line 0, column 0)
      t1 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.