8000 Inconsistent isolation of top-level closure · Issue #81599 · swiftlang/swift · GitHub
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Inconsistent isolation of top-level closure #81599

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sutaner opened this issue May 19, 2025 · 1 comment
Open

Inconsistent isolation of top-level closure #81599

sutaner opened this issue May 19, 2025 · 1 comment
8000
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bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. triage needed This issue needs more specific labels

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@sutaner
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sutaner commented May 19, 2025

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
@sutaner sutaner added bug A deviation from expected or documented behavior. Also: expected but undesirable behavior. triage needed This issue needs more specific labels labels May 19, 2025
@rayx
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rayx commented May 20, 2025

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
  }

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