[SR-15745] Actor methods able to be called from escaping closure without `await`

[CharlesJS]

Previous ID	SR-15745
Radar	rdar://90354322
Original Reporter	@CharlesJS
Type	Bug

Environment

$ swiftc --version
swift-driver version: 1.26.21 Apple Swift version 5.5.2 (swiftlang-1300.0.47.5 clang-1300.0.29.30)
Target: x86_64-apple-macosx12.0

Additional Detail from JIRA

Votes	3
Component/s	swift
Labels	Bug
Assignee	None
Priority	Medium

md5: 581829c2432961ebf61e130142bf32e5

Issue Description:

This compiles, and I'm pretty sure it shouldn't:

import Dispatch

func someFunctionThatDoesWhoKnowsWhatWithThis(closure: @escaping () -> ()) {
  DispatchQueue.main.async { closure() }
}

class SomeClass {}

actor Foo {
  var bar = SomeClass()

  func setBar(_ bar: SomeClass) { self.bar = bar }
 
  func baz() {
    someFunctionThatDoesWhoKnowsWhatWithThis {
      self.setBar(.init()) // shouldn't this require an `await`?
    }
  }
}

The code inside the `@escaping` closure is no longer running inside the actor's context, so it seems like it shouldn't be able to call the actor's methods without an `await`, and that this is therefore a bug.

Forum thread with related discussion: https://forums.swift.org/t/calling-escaping-closure-from-actor-context-why-dont-i-need-await-here/51029

[DougGregor]

As recently noted in the forum thread, this is an artifact of the compiler suppressing Sendable diagnostics in code that hasn't adopted concurrency. You can turn on completely checking manually with -Xfrontend -warn-concurrency (in Swift 5.6) or Swift 5.7's upcoming -strict-concurrency=complete. Doing so with my local Swift 5.7 build produces this:

t2.swift:4:30: warning: capture of 'closure' with non-sendable type '() -> ()' in a @Sendable closure
  DispatchQueue.main.async { closure() }
                             ^
t2.swift:4:30: note: a function type must be marked '@Sendable' to conform to 'Sendable'
  DispatchQueue.main.async { closure() }
                             ^

as it should.