Effects of explicit nonisolated on isolation inference

Using Swift
1

I’m having difficulty explaining some behavior I’ve noticed. The following code compiles. But if you mark either protocol as nonisolated, it appears to cut off the MainActor isolation inference for MyClass.requirement.

I find this a little surprising when applied to A, and very surprising when applied to B.

// default: nonisolated

protocol A {
	func requirement()
}

protocol B: A {
}


@MainActor
final class MyClass: @MainActor B {
	var state = 1

	init() {
	}

	func requirement() {
		// if either protocol is explicitly nonisolated:
		// Error: Main actor-isolated property 'state' can not be referenced from a nonisolated context
		print(state)
	}
}

This came up in the context of using default MainActor, which can motivate adding explicit nonisolated where you otherwise wouldn’t need it.

I’ve re-read SE-0449, but I wasn’t able to find a clear justification for this behavior. Did I miss something?

2 Likes
2

not an answer, but to further add to the confusion... indirecting through another protocol refinement produces different behavior (which also makes me think there is a bug involved somewhere):

protocol A {
	func requirement()
}
protocol B: A {}
nonisolated protocol C: B {}

@MainActor
final class MyClass: @MainActor C
{
	var state = 1
	init() {}

	func requirement() {
		print(state) // ✅ – still inferred to be @MainActor
	}
}

also, if you declare the conformance via an extension, it appears to behave differently:

protocol A {
	func requirement()
}
nonisolated protocol B: A {}

@MainActor
final class MyClass
{
	var state = 1
	init() {}

	func requirement() {
		print(state) // ✅
	}
}

extension MyClass: @MainActor B {}

i find myself currently struggling to formulate how i think these feature interactions even should work... do you feel you have a sense for that? my intuition is that the explicit isolated conformance being written on the class declaration should 'get priority' over the protocols being marked nonisolated.

cc @hborla @Douglas_Gregor – curious if either of you could shed light on what's happening in these cases, or what the intended interaction between isolated conformances and 'global actor inference cutoff' should be.

5 Likes