And/or in arguments

After a long talk with defr and bojanz in Paris, I would like to raise this issue officially so we can evaluate and coordinate the work needed to achieve this.

Arguments and filters are basically the same. They are used to reduce the amount the results of the query by adding WHERE statements. But arguments are a bit more complex because they can come from the URL / be dynamic, and can (or should) be validated. 

Complexity put aside, arguments produce the same effects on the query. So it should be natural to combine them the same way, to be able to and/or them. 

Durung the long talks I had, several non trivial issues were raised, related the superior complexity of arguments. 

1. Arguments can be validated, so what happens when an argument that is "or-ed" does not validate ? Example : nid is argument 1, tid is argument 2 , both coming from URL. They are "or-ed". tid doesn't validate because it's not from the proper vocabulary. The term validator states that it shows the 404 page (or whatever option) when it returns false. But the nid validator returns right. Should this page show only the results according to its nid, or take into account the tid validator ? Because from the user point of view, one may want to show results when only one of them is present (this is the second big issue), but display 404 when they doesn't not validate ; or display results anyway. 

2. Second issue is related to optional arguments. When "or-ed", an argument that is not present changes the ordering of the arguments in the URL, and so can break validation. Same example than above : nid is %1, tid is %2, nid is not there. How to tell Views to validate tid which is now %1 as if it was %2 ?

3. This issue is just the logic sequel to 1 and 2. Views has several options about how to handle cases when arguments is not present, and when it does not validate. With and/or possibility, we need a proper way and/or a consensus about how to handle all combinations possible, or a beautiful rule, AND a well thought UI. 

4. This well thought UI. Arguments' complexity means the UI can grow far more complex. What I know for sure is that there will be more than what is in the filters section of the views UI (and/or select list). The UI will depend on the way chosen to handle the combinations. Will we have a global validator ? A global action for when all arguments validates or not ? Only those "or-ed", or "and-ed", or manually selected with a checkbox ? A refactored arguments section to make it right ?

5. UI again. Acquia recently sponsored a new UI for the d7 branch. What about it if further changes needs to be done related to arguments ?

6. I haven't talked about feasibility or technical problems, because I'm not knowledgeable enough about Views to do it. Maybe it is completely, or partially only doable. Views maintainers or activists input required here.