swiftlang/swift

global actor annotations can be applied to inheritance clauses

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#86,693 opened on Jan 21, 2026

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 (3 comments) (0 reactions) (0 assignees)Swift (10,719 forks)batch import
accepts invalidactor isolationattributesconcurrencygood first issueinheritance clausesswift 6.3type checker

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Description

Description

This code is, as far as I understand, meaningless. It should not be accepted, even though I think it is just a no-op. I bet it is now parsed because of isolated conformances but then just silently dropped further down the line.

Reproduction

class Base {
}

class Derived: @MainActor Base {
}

Expected behavior

This should produce an error.

Environment

Apple Swift version 6.3-dev (LLVM d402ab9729f318e, Swift 2de856568572fc0) Target: arm64-apple-macosx26.0

Additional information

No response

Contributor guide

Research direction
Investigate why @MainActor applied to inheritance clause in class Derived is not rejected by the parser. Trace the parsing of inheritance clauses and see how global actor annotations are handled, particularly in the context of isolated conformances. Look at the AST and type checking logic that silently drops such annotations.
Tech stack
swift
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Mostly clear
Prerequisites
Swift
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35