Runtime-safe array subscripting

proposals/XXXX-runtime-safe-array-subscripting.md

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+# Runtime-safe array subscripting
+
+* Proposal: [SE-NNNN](https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md)
+* Author(s): [Rudolf Adamkovič](https://github.com/salutis)
+* Status: **Awaiting review**
+* Review manager: TBD
+
+## Introduction
+
+Add an alternative way to subscript arrays where out-of-bounds runtime errors are converted to `nil`.
+
+Swift-evolution thread: [Optional safe subscripting for arrays](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160111/006826.html)
+
+## Proposed solution
+
+Allow to safely index into arrays returning `Element?`:
+
+```swift
+// Sample data
+var array = [0, 1, 2]
+
+// Setting values
+array[ifExists: 0] // same as array[0]
+array[ifExists: 3] // out of bounds, evaluates to nil
+
+// Getting values
+array[ifExists: 0] = 42 // same as array[0] = 42
+array[ifExists: 3] = 42 // out of bounds, no operation
+```
+
+## Impact on existing code
+
+No existing code is affected.
+
+## Alternatives considered
+
+* Extend the `CollectionType` or `Indexable` protocol instead the `Array`