[SR-3956] In Swift 3.1, assigning a let protocol's var in init generates an error

[swift-ci]

Previous ID	SR-3956
Radar	None
Original Reporter	benasher44 (JIRA User)
Type	Bug

Environment

macOS Sierra 10.12.3
Xcode 8.2.1 (App Store version)
Swift 3.1 snapshot: 3.1-DEVELOPMENT-SNAPSHOT-2017-02-11-a

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, 3.1Regression
Assignee	None
Priority	Medium

md5: 7117b5bd4615c0cd1a5a3e3e77ecfedd

Issue Description:

The following code works fine in Swift 3 (3.0.2) but generates an error in Swift 3.1:

protocol SampleDelegate: class {}

protocol Sample {
    weak var delegate: SampleDelegate? { get set }
}

class SampleA: Sample {
    weak var delegate: SampleDelegate?
}

class Sampler: SampleDelegate {
    let s: Sample = SampleA()

    init() {
        self.s.delegate = self
    }
}

Here is the generated error:

test.swift:15:25: error: cannot perform mutating operation: 'self.s' is a 'let' constant
self.s.delegate = self
^
test.swift:12:5: note: change 'let' to 'var' to make it mutable
let s: Sample = SampleA()
^~~
var

I can see why this was allowed before, since the let constant doesn't need to be guaranteed to be fully initialized until after init has completed.

[belkadan]

Hm. @jckarter, what do you think?

benasher44 (JIRA User), note that if you make Sample class-constrained as well the problem goes away. As written, the error is arguably correct, since setting anything on a let property, ever, would be invalid if it might be a value type.

[swift-ci]

Comment by Ben A (JIRA)

That makes sense

[belkadan]

Oh, yes, we definitely want to know about it. :-) Thanks for reporting.