swiftlang/swift

Error says `@preconcurrency` only applies in inheritance clauses

Open

#88338 opened on Apr 7, 2026

View on GitHub
 (2 comments) (0 reactions) (0 assignees)Swift (69,989 stars) (10,719 forks)batch import
@preconcurrencyTypeResolverattributescompilergood first issueswift 6.3type checker

Description

Description

This is not true. The attribute can also be used on declarations such as types, functions, and imports.

Reproduction

let _: @preconcurrency @MainActor() -> Void = {}

error: '@preconcurrency' only applies in inheritance clauses
  83 | 
  84 | let _: @preconcurrency @MainActor() -> Void = {}
     |        `- error: '@preconcurrency' only applies in inheritance clauses
  85 |

Expected behavior

An accurate error message. It might be better to say that @preconcurrency cannot be used on a type attribute instead of enumerating all the valid positions.

Environment

Swift version 6.3-dev (fefcb257b5b5d32)

Additional information

No response

Contributor guide

Tech stack
swift
Domain
developer experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
None
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
Search the Swift compiler source code for the exact error message 'only applies in inheritance clauses' to locate the diagnostic. The message is likely in a diagnostics file (e.g., DiagnosticsFrontend.def). Once found, modify the string to state that @preconcurrency cannot be used on a type attribute, as suggested in the issue. Ensure the fix is consistent with other diagnostic messages. Then update any existing test cases that reference the old error message to expect the new one.