Error found:
in module ClassHeadNoVTA7
at tests/purs/failing/ClassHeadNoVTA7.purs:12:8 - 12:26 (line 12, column 8 - line 12, column 26)

  No type class instance was found for
                                
    ClassHeadNoVTA7.TestClass t1
                              t2
                                
  The instance head contains unknown type variables. Consider adding a type annotation.

while applying a function testMethod
  of type TestClass @t0 t1 t2 => Maybe t1 -> Int
  to argument Nothing
while checking that expression testMethod Nothing
  has type Int
in value declaration test

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.