Error found:
in module Main
at tests/purs/failing/3531-3.purs:22:11 - 22:22 (line 22, column 11 - line 22, column 22)

  No type class instance was found for
                          
    Main.C (X             
              { foo :: Int
              | t1        
              }           
              { foo :: 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

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