5211: Fix inference of indexing argument (partly) r=flodiebold a=flodiebold
We need to add the `T: Index<Arg>` obligation to be resolved later as well, otherwise we can't make inferences about `Arg` later based on the `Index` impls.
This still doesn't fix indexing with integer variables though; there's a further problem with Chalk floundering because of the variable, I think.
Co-authored-by: Florian Diebold <florian.diebold@freiheit.com>
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