CLASS question

The code below does not compile with XE 17 (probably justly so). It compiles if U and V are not declared as allocatable arrays.The help for CLASS in the provided documentation does not seem to cover this case, maybe I am missing something here?

PROGRAM P
IMPLICIT NONE
TYPE,ABSTRACT :: T
END TYPE T
TYPE,EXTENDS(T) :: T1
END TYPE T1
TYPE(T1),ALLOCATABLE :: U(:)
CALL S(U)
CONTAINS
SUBROUTINE S(V)
IMPLICIT NONE
CLASS(T),ALLOCATABLE :: V(:)
END SUBROUTINE S
END PROGRAM P