Side-effects in StlDiscreteTimeOfflineAstVisitor

[ManCla]

In the STL discrete-time offline AstVisitor some of the visit functions manipulate the variable returned by the recursive call to self.visit(node.children[0], *args, **kwargs) (most times called sample). However, this is the very same variable that was stored in the result field of the ast at line 14. Therefore, the changes performed in the visit functions will cause side-effects in the ast.result dictionary.

In visitVariable even the input traces are directly returned and might be changed by other functions.

Solution is either to change the visit function at line14 to store a copy of the variables in ast.results or to avoid all changes to the sample variable in the various visit functions. Probably best to do both?

I have not checked if other interpreters have the same issue.

[ManCla]

The functions that contain in-place computations are (list made with the help of copilot, manually checked):

• visitTimedAlways : Uses sample += [float('inf')] * (...) which mutates the list object returned by the child visit in-place. If that child returned a list that is shared/aliased (e.g. original variable list or a previously stored result), this will mutate that shared list.
• visitTimedEventually : Uses sample += [-float('inf')] * (...) — same in-place mutation risk as above.
• visitVariable : In the no-field branch it returns the original list from self.ast.var_object_dict (sample_return = var) rather than a copy. That exposes the original list to callers — if any visitor later mutates that list in-place (for example via += in the timed functions), the original input data in ast.var_object_dict will be mutated.

[ManCla]

since it's only a couple of them, it makes sense to fix them and do not bother too much about efficiency. I fixed them in the PR.