Inconsistent isolation of top-level closure

[sutaner]

Description

It seems that a top-level closure without attributes is interpreted as both @MainActor and non-@MainActor.

Reproduction

enum Something {

    @MainActor static var value = 0

}

let closure: () -> Void = {
    Something.value += 1
}

let sendableClosure: @Sendable () -> Void = {
    closure()
}

See also: https://forums.swift.org/t/why-can-a-sendable-closure-call-a-non-sendable-closure/79921

Expected behavior

The given code compiles (even in language mode 6) although it should not.

Environment

swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
Target: arm64-apple-macosx15.0

Additional information

No response

[rayx]

Workaround: declaring closure()'s isolation explicitly helps compiler to catch the issue.

  enum Something {
      @MainActor static var value = 0
  }

- let closure: () -> Void = {
+ let closure: @MainActor () -> Void = {
      Something.value += 1
  }

  let sendableClosure: @Sendable () -> Void = {
      closure() // error: call to main actor-isolated let 'closure' in a synchronous nonisolated context
  }