Function Overload

Hi, everyone. There is a problem a I'm not quite understand about optional dummy argument. Here is the code:

MODULE Test
    IMPLICIT NONE

    TYPE :: MyType
        INTEGER :: a
    CONTAINS
        GENERIC, PUBLIC :: Set => SetBy2Arguments,&
                                  SetBy3Arguments
        PROCEDURE, PASS :: SetBy2Arguments
        PROCEDURE, PASS :: SetBy3Arguments
    END TYPE MyType

    CONTAINS

    FUNCTION SetBy2Arguments(this, arg1, arg2)
        IMPLICIT NONE

        INTEGER :: SetBy2Arguments
        CLASS(MyType), INTENT(IN) :: this
        INTEGER,       INTENT(IN) :: arg1
        INTEGER,       INTENT(IN) :: arg2

        SetBy2Arguments = this%a + arg1 + arg2

        RETURN
    END FUNCTION SetBy2Arguments

    FUNCTION SetBy3Arguments(this, arg1, arg2, arg3)
        IMPLICIT NONE

        INTEGER :: SetBy3Arguments
        CLASS(MyType),     INTENT(IN) :: this
        INTEGER,           INTENT(IN) :: arg1
        INTEGER,           INTENT(IN) :: arg2
        INTEGER, OPTIONAL, INTENT(IN) :: arg3

        SetBy3Arguments = this%a + arg1 + arg2 + arg3

        RETURN
    END FUNCTION SetBy3Arguments

END MODULE Test

When compiling, The compiler generates a warning message saying that:

warning #8449: The type/rank/keyword signature for this specific procedure matches another specific procedure that shares the same generic binding name.   [SETBY3ARGUMENTS]

So why are these 2 binding procedures matches each other ? And How is the compiler treat a dummy argument that has an OPTIONAL attribute ? Appriciate any replies, thanks.