Recursive DataStructures must be validated and reported as a problem

[riederm]

Recursive datastructures cannot be declared in ST. We need a validation to permit users from declaring such datastructures using STRUCTS, FB and CLASSES.

example:

TYPE A: STRUCT
     other: B;
END_STRUCT
END_TYPE

TYPE B: STRUCT
     other: A;
END_STRUCT
END_TYPE

Such recursions can be immediate (a struct containing itself) but they can also be formed over numerous hops (A->B->...->H->A).

We basically need to show that the declaration-graph (starting from all global variables and all programs) contains no cycles.

Every cycle should be reported as a problem.

Note that POINTER-declarations must not be treated as edges in the declaration-graph.