[SR-8951] Adding an init with matching type of another init should error with ambiguous reference

[swift-ci]

Previous ID	SR-8951
Radar	None
Original Reporter	benasher44 (JIRA User)
Type	Bug

Environment

macOS 10.13.6
Swift 4.2
Xcode 10.0 (10A255)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, 4.0Regression
Assignee	None
Priority	Medium

md5: d8d439d344924ff347951eb1e0ab863e

Issue Description:

I went to add an extension to RangeReplaceableCollection today, and some tests started failing because some String s in our codebase started behaving funny. It turns out that a flatMap picked a different init without warning, which changed behavior and failed our tests.

Consider the following code:

import Foundation

//extension RangeReplaceableCollection {
//    public init(minimumCapacity: Int) {
//        self.init()
//        self.reserveCapacity(minimumCapacity)
//    }
//}

let num: Int? = 100
let str = num.flatMap(String.init) ?? ""

print("|\(str)|")

As is, this code prints |100|, but if you un-comment the extension, it prints ||. I would expect uncommenting the extension to cause a compiler error because that would make String.init ambiguous.

[swift-ci]

Comment by Ben A (JIRA)

I worked around this by constraining the extension:

extension RangeReplaceableCollection where Self: ExpressibleByArrayLiteral

[swift-ci]

Comment by Ben A (JIRA)

In trying figure out if/when this regressed, I changed the example a bit (String wasn't a RangeReplaceableCollection until Swift 4):

extension String {
    public init(minimumCapacity: Int) {
        self.init()
    }
}

let num: Int? = 100
let str = num.flatMap(String.init) ?? ""

print("|\(str)|")

This tweaked sample has the same issue in Swift 4.0 (prints differently depending on the presence of this extension), but it errors as expected in Swift 3.1:

error: ambiguous use of 'init'

[AnnaZaks]

cc @airspeedswift, @DougGregor, @slavapestov

[swift-ci]

Comment by Ben A (JIRA)

Apologies for the overly-specific title. If someone has a better understanding of the impact here and how it likely spans beyond inits, feel free to update it!