[SR-15916] @MainActor-isolated default arguments for @MainActor-isolated functions

[groue]

Previous ID	SR-15916
Radar	None
Original Reporter	@groue
Type	Bug

Environment

Xcode 13.3 beta 3 (13E5104i)

Additional Detail from JIRA

Votes	2
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 618b69843e92ff0edf9587c87894767b

Issue Description:

Hello,

The following snippet does not compile:

@MainActor class Foo { }

// error: call to main actor-isolated initializer 'init()' in a synchronous nonisolated context
// @MainActor func f(foo: Foo = Foo()) { }
//                               ^
// note: calls to initializer 'init()' from outside of its actor context are implicitly asynchronous
// @MainActor class Foo {
//                  ^
@MainActor func f(foo: Foo = Foo()) { }

It should compile, since everything in the declaration of the f function is MainActor-isolated.

[groue]

Related thread: https://forums.swift.org/t/using-mainactor-values-in-initializer-default-arguments/52694

[alberussoftware]

Such an annoying bug. Are there any rough plans for when it will be fixed? @ktoso @DougGregor

[vykut]

@AnthonyLatsis are there plans to fix this bug in Swift 5.9?

[AnthonyLatsis]

I have a feeling fixing this might warrant a formal evolution proposal that outlines the semantics and restrictions for public approval, so I suppose not, but we should really ask the concurrency experts (@ktoso or @DougGregor ping).

[ktoso]

Might need a mini proposal, let me track it with a radar. rdar://112171197

@hborla FYI