Please find the related proposal which was formed after the long discussion in the list:

https://github.com/luish/swift-evolution/blob/proposal-lenient-collection-subscripts/proposals/nnnn-more-lenient-collections-subscripts.md

Here is the pull request on the swift-evolution repo:
https://github.com/apple/swift-evolution/pull/328

On 23.06.2016 13:12, Andreas Ley via swift-evolution wrote:
(First time using a mailing list; I hope this message ends up in the correct 
thread)

This is a topic that comes up regularly on the Swift evolution mailing list and 
off it.
After reading through all the respective threads again, there seem to be the 
following two camps:

Arguments made for crashing when accessing a non-existent index:
- fast
- shows bugs quickly

Arguments made in favor of returning an optional by default:
- safe (as in "doesn't crash")
- similar to what other modern languages do
- what an unexperienced Swift developer would expect

All are valid arguments, but for different use cases.
In my opinion, the biggest problem is that there's no indication that 
subscripting can crash on the default array. Alternative subscripts for bounded 
access wouldn't solve this either.

Maybe Swift should have two different array classes: A fast, fast-failing "UnsafeArray" and a 
default safe "Array". This would prevent unexpected crashes for new Swift programmers while still 
providing a faster alternative for those who do low-level stuff. The name "UnsafeArray" would 
clearly communicate that this class should be handled with care.

- Andreas
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