Error found:
in module Main
at tests/purs/failing/3531-2.purs:22:11 - 22:22 (line 22, column 11 - line 22, column 22)

  No type class instance was found for
                     
    Main.C (X t2 Int)
                     
  The following instance partially overlaps the above constraint, which means the rest of its instance chain will not be considered:

    Main.cx

  The instance head contains unknown type variables. Consider adding a type annotation.

while applying a function thing
  of type C t0 => t0 -> t0
  to argument test1
while inferring the type of thing test1
in value declaration test2

where t0 is an unknown type
      t1 is an unknown type
      t2 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.