type_param.scala

scala> class A[M[_]]
warning: there was one feature warning; re-run with -feature for details
defined class A

scala> class B[M[+_]]
warning: there was one feature warning; re-run with -feature for details
defined class B

scala> class C[M[-_]]
warning: there was one feature warning; re-run with -feature for details
defined class C

///////////////////////////////////////////////////////

scala> trait X[T]
defined trait X

scala> trait Y[+T]
defined trait Y

scala> trait Z[-T]
defined trait Z

///////////////////////////////////////////////

scala> new A[X]
res12: A[X] = A@2b0c189b

scala> new A[Y]
res13: A[Y] = A@333038c7

scala> new A[Z]
res14: A[Z] = A@75dd085d

////////////////////////////////////////////////

scala> new B[X]
<console>:9: error: kinds of the type arguments (X) do not conform to the expected kinds of the type parameters (type M) in class B.
X's type parameters do not match type M's expected parameters:
type T is invariant, but type _ is declared covariant
       val res18 =
           ^
<console>:10: error: kinds of the type arguments (X) do not conform to the expected kinds of the type parameters (type M) in class B.
X's type parameters do not match type M's expected parameters:
type T is invariant, but type _ is declared covariant
              new B[X]
                  ^

scala> new B[Y]
res19: B[Y] = B@1a5c786a

scala> new B[Z]
<console>:9: error: kinds of the type arguments (Z) do not conform to the expected kinds of the type parameters (type M) in class B.
Z's type parameters do not match type M's expected parameters:
type T is contravariant, but type _ is declared covariant
       val res20 =
           ^
<console>:10: error: kinds of the type arguments (Z) do not conform to the expected kinds of the type parameters (type M) in class B.
Z's type parameters do not match type M's expected parameters:
type T is contravariant, but type _ is declared covariant
              new B[Z]
                  ^
                  
///////////////////////////////////////////////////////////                  

scala> new C[X]
<console>:9: error: kinds of the type arguments (X) do not conform to the expected kinds of the type parameters (type M) in class C.
X's type parameters do not match type M's expected parameters:
type T is invariant, but type _ is declared contravariant
       val res21 =
           ^
<console>:10: error: kinds of the type arguments (X) do not conform to the expected kinds of the type parameters (type M) in class C.
X's type parameters do not match type M's expected parameters:
type T is invariant, but type _ is declared contravariant
              new C[X]
                  ^

scala> new C[Y]
<console>:9: error: kinds of the type arguments (Y) do not conform to the expected kinds of the type parameters (type M) in class C.
Y's type parameters do not match type M's expected parameters:
type T is covariant, but type _ is declared contravariant
       val res22 =
           ^
<console>:10: error: kinds of the type arguments (Y) do not conform to the expected kinds of the type parameters (type M) in class C.
Y's type parameters do not match type M's expected parameters:
type T is covariant, but type _ is declared contravariant
              new C[Y]
                  ^

scala> new C[Z]
res23: C[Z] = C@3ffe9d93