[SR-5515] popFirst error message is silly

[mattneub]

Previous ID	SR-5515
Radar	None
Original Reporter	@mattneub
Type	Bug

Environment

Xcode Version 9.0 beta 3 (9M174d)

Additional Detail from JIRA

Votes	2
Component/s	Compiler
Labels	Bug, DiagnosticsQoI, TypeChecker
Assignee	@xedin
Priority	Medium

md5: a87fa35be4ed517d2c1b701a237232de

Issue Description:

var arr = [1,2,3]
arr.popFirst()  // cannot use mutating member on immutable value: 'arr' is immutable

That's a silly error message. `arr` is not immutable in any normal sense. The problem is actually that popFirst doesn't operate on an Array at all (something I've always found rather weird, but there it is).

[belkadan]

Yeah, we're looking at this popFirst.

@xedin, any idea what's going on here? And @dabrahams, why doesn't RangeReplaceableCollection have its own popFirst?

[dabrahams]

Pop operations are supposed to be O(1), and many models of RangeReplaceableCollection can't offer that guarantee.

[belkadan]

Ah, of course. Thanks.