Misleading error about `nonisolated` not being applicable to mutable stored properties

Description

@MainActor
final class C {
  nonisolated var c = 0
}
actor A {
  nonisolated var a = 0
}
@MainActor
struct S {
  nonisolated var s = 0
}

<source>:3:3: error: 'nonisolated' cannot be applied to mutable stored properties
 1 | @MainActor
 2 | final class C {
 3 |   nonisolated var c = 0
   |   |               `- note: convert 'c' to a 'let' constant or consider declaring it 'nonisolated(unsafe)' if manually managing concurrency safety
   |   `- error: 'nonisolated' cannot be applied to mutable stored properties
 4 | }
 5 | actor A {

<source>:6:3: error: 'nonisolated' cannot be applied to mutable stored properties
 4 | }
 5 | actor A {
 6 |   nonisolated var a = 0
   |   |               `- note: convert 'a' to a 'let' constant or consider declaring it 'nonisolated(unsafe)' if manually managing concurrency safety
   |   `- error: 'nonisolated' cannot be applied to mutable stored properties
 7 | }
 8 | @MainActor

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.

4

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.

blocked

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Swift concurrencycompiler diagnosticsAST basics

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

30

Research direction

The error message is emitted in the Swift compiler's diagnostic code. Locate where 'nonisolated cannot be applied to mutable stored properties' is defined (likely in lib/Sema/TypeCheckConcurrency.cpp or similar). The issue reports that the error is incorrect for value types like struct S; the diagnostic should be refined to only apply to classes and actors. Check the AST for the declaration context and adjust the error condition to exclude value types. The recent comments may provide additional guidance.