a.f90

program XX

requirement is_binary_op(T, op)
    type, deferred :: T
    function op(lhs, rhs) result(res)
        type(T), intent(in) :: lhs, rhs
        type(T) :: res
    end function
end requirement


 TEMPLATE my_template(T, OP)
    TYPE, DEFERRED :: T
    REQUIRE :: is_binary_op(T, OP)
 CONTAINS
    function reduce(x) result(y)
       TYPE(T) :: y
       TYPE(T), INTENT(IN) :: x(:)
       INTEGER :: i
       y = x(1)
       DO i = 2, SIZE(x)
          y = OP(y, x(i))
       END DO
    END function reduce
 END TEMPLATE

INSTANTIATE my_template(REAL, OPERATOR(+))
!INSTANTIATE MY_TEMPLATE(MY_T, MY_PLUS)

real :: x(4)
real :: y
x = [1., 2., 3., 4.]
y = reduce(x)
print *, x
print *, y

end