If you come from denzil to for instance dylan, you might get weird issues about tab/space as the enforcement was introduced in the meantime, perhaps even for good. However, there are two issues in here: 1) You will get an error report for your recipe even if the error is inside the .inc required! 2) You will not get proper location, just a file name. It would need more fine tuning for better user experience. I have been told the parser design is not prepared for it. That does not mean it should not be fixed. It just means that fixing the bad design might take longer.
(2) isn't true, you get a function name. Generally the problem isn't specific to any particular line but the entire function.
I do not consider a function proper. Especially when code gets pasted from somewhere in a more fine-tuned manner than a function. I do consider it true.
The issue is that the function can consist of expanded data. We therefore can't check at the point the function is first created since it can be subsequently modified. We do have variable tracking code however we default to not running with it since it has significant performance overhead. We therefore have to make a design choice between the usability aspect and performance. The cases where people run into that error are comparatively rare, most people have migrated from denzil at this point. We therefore chose to keep the performance and won't be able to fix this issue any time soon.