assumed-size (C071)

This rule is turned on by default.

What does it do?

Checks for assumed size variables

Why is this bad?

Assumed size dummy arguments declared with a star * as the size should be avoided. There are several downsides to assumed size, the main one being that the compiler is not able to determine the array bounds, so it is not possible to check for array overruns or to use the array in whole-array expressions.

Instead, prefer assumed shape arguments, as the compiler is able to keep track of the upper bounds automatically, and pass this information under the hood. It also allows use of whole-array expressions, such as a = b + c, where a, b, c are all arrays of the same shape.

Instead of:

subroutine process_array(array)
    integer, dimension(*), intent(in) :: array
    ...

use:

subroutine process_array(array)
    integer, dimension(:), intent(in) :: array
    ...

Note that this doesn't apply to character types, where character(len=*) is actually the most appropriate specification for intent(in) arguments! This is because character(len=:) must be either a pointer or allocatable.