[SR-9770] Singleton Initializer can enter Infinite Loop

[swift-ci]

Previous ID	SR-9770
Radar	https://bugreport.apple.com/web/?problemID=47576365
Original Reporter	spamsencer (JIRA User)
Type	Bug

Environment

Swift 4.2

Xcode 10.1 (10B61)

macOS 10.14.2 (18C54)

Additional Detail from JIRA

Votes	0
Component/s	Compiler, LLDB for Swift
Labels	Bug
Assignee	None
Priority	Medium

md5: 265449bf1b4b45378b6f0bec0aede923

Issue Description:

When creating a Swift Singleton object as follows:

static let sharedHandler = SomeHandler()

If the singleton initializes top-level variables at the time of initialization, and if those variables' initializers reference the singleton object, the execution thread enters an infinite loop.

[belkadan]

I don't think there's a fix for this, but maybe we could at least detect the infinite loop in debug builds or something, so we get a more reliable error?

[swift-ci]

Comment by Samuel Spencer (JIRA)

Agreed – seems like something that could potentially be caught / warned against at build time. Any suggestions on where I might go looking around to make a contribution for this?

[belkadan]

@swift-ci create