Make sure a method returns something.
Make it a type check error.

For now, as basic blocks are not handled yet, we cannot do dataflow analysis,
so we cannot easily make sure return statement are present in all possible traces.

Well, this one should be adapted with future method syntax, where a tuple can be passed as out parameter. In this syntax, the return statement dissapear.
To adapt this task, the dataflow analysis should make sure all variables of the tuple are assigned before exiting method.