[SR-16009] MainActor static-let initialization not enforced on main thread.

[kavon]

Previous ID	SR-16009
Radar	rdar://problem/90436577
Original Reporter	@kavon
Type	Bug

Environment

swift-driver version: 1.26.9 Apple Swift version 5.5 (swiftlang-1300.0.31.1 clang-1300.0.29.1)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, Concurrency
Assignee	@kavon
Priority	Medium

md5: f6b9b747f9f450df4eddd1f7c0fe8e4f

cloned from:

• [SR-15227] MainActor static variable initialization not enforced on main thread. #57549

Issue Description:

@MainActor 
struct Bar {
  init() { 
    print("bar init is main", Thread.isMainThread) 
  } 
  func barCall() { 
    print("bar call is main", Thread.isMainThread) 
  } 
} 

@MainActor 
struct Foo { 
  @MainActor 
  static let global = Foo() 

  init() { 
    print("foo init is main", Thread.isMainThread) 
    let b = Bar() 
    b.barCall() 
  } 

  func fooCall() { 
    print("foo call is main", Thread.isMainThread) 
  } 

} 

Task.detached { 
  await Foo.global.fooCall() 
} 
// prints: 
// foo init is main false 
// bar init is main false 
// bar call is main false 
// foo call is main true

While fooCall() is enforced on the main thread, the initialization of the static variable is not, which breaks the contract of @MainActor.

[kavon]

@swift-ci create

[moreindirection]

This is still present in Swift 5.8

[AnthonyLatsis]

Revision update: Swift version 5.9-dev (LLVM 91080e65df791d6, Swift c1d5118)

[bradleymackey]

This is still present in Swift 5.9