#index is not correct for filter-map sequences #758
Description
The #index pointer in filter map sequences is always incremented in map, even though some elements have been skipped.
I believe this is caused by hte filter map optimization pass. Since map(filter(..)) are compiled as a single loop, the #index pointer has the same value in both the filter and the map predicate when they are evaluated.
This means that we get different results if filter maps are directly chained, or if they are done in two steps with variable assignements.
Btw, the explain tab in the playground seems to give the correct results.
Example:
// chained
let a = 0..5 | filter(# %2 == 0) | map(#index);
// not chained
let b_filter = 0..5 | filter(# %2 == 0);
let b_map = b_filter | map(#index);
[a, b_map]
Output
[[0,2,4], [0,1,2]]
Expected output
[[0,1,2], [0,1,2]]