As discussed here, a current pain-point with no-unnecessary-condition is that array indexes are often a source of false positives because the types don't represent the possibility of an out-of-bounds error.

This adds a check to ignore code like:

functionexample(arr:number[]){if(arr[0]){}}

This version only handles the "obvious" case where the index signature is directly inside the condition - it will still raise errors on code like:

functionexample(arr:number[]){consthead=array[0];if(head){}}

I believe fixing this would be possible, but I'm not sure it's worth the complexity of implementation.

I considered making this configurable, but I just can't come up with any cases in which you'd actually want the previous behavior. Unlike the object case, I don't think anyone is realistically going to model their arrays asArray<T | undefined> for the sake of safe index checks.

This I don't think this is considered a breaking-change - all previously valid code is still valid - but if so, I'd rather point at 3.0 than put it behind a temporary option flag.

Fixes#1544