swiftlang/swift

global actor annotations can be applied to inheritance clauses

Open

#86693 opened on Jan 21, 2026

View on GitHub
 (2 comments) (0 reactions) (0 assignees)Swift (69,989 stars) (10,719 forks)batch import
accepts invalidactor isolationattributesconcurrencygood first issueinheritance clausesswift 6.3type checker

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

Tech stack
swift
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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.
clear
Prerequisites
understanding of Swift concurrencyknowledge of compiler parsing
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the Swift compiler's parsing of inheritance clauses, specifically how global actor annotations like @MainActor are applied. The issue suggests the annotation is parsed but then silently dropped. Look at the parser code for class declarations (e.g., in lib/Parse/ParseDecl.cpp) and the semantic analysis (e.g., in lib/Sema/TypeCheckDecl.cpp) to see where the error should be emitted. Add a diagnostic to reject such meaningless annotations.