[swift-evolution] Why doesn't removeLast() on Collection return an optional?

[Louis D'hauwe]

Regarding the removeLast() function on Collection:

The current implementation <https://github.com/apple/swift/blob/c3b7709a7c4789f1ad7249d357f69509fb8be731/stdlib/public/core/BidirectionalCollection.swift#L228> is:
@discardableResult
public mutating func removeLast() -> Iterator.Element {
 	let element = last!
	self = self[startIndex..<index(before: endIndex)]
	return element
}

This makes it so that if you call removeLast() on an empty collection you get a fatal error.
("fatal error: can't remove last element from an empty collection")

The documentation for removeLast() <https://github.com/apple/swift/blob/c3b7709a7c4789f1ad7249d357f69509fb8be731/stdlib/public/core/BidirectionalCollection.swift#L220> even has this noted:
"The collection must not be empty.".

Surely you could just remove the explicit unwrapping of 'last' and add a guard statement? 

As such:
@discardableResult
public mutating func removeLast() -> Iterator.Element? {
 	guard let element = last else {
		return nil
	}
	self = self[startIndex..<index(before: endIndex)]
	return element
}

It sure seems more "Swifty" to alert at compile time that removing the last item of a collection might fail, and make it return nil as a result.

– Louis D'hauwe

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