@Observable macro fails on Linux: 'shouldNotifyObservers' not covered by macro (Swift 6.2.3)

[guitaripod]

Description

Building any project using @Observable on Linux with Swift 6.2.3 fails with macro expansion errors.

Environment

• OS: Arch Linux (kernel 6.18.3-arch1-1)
• xtool: 1.16.1
• Swift: 6.2.3 (swift-6.2.3-RELEASE)
• iOS SDK: darwin (iOS 18.5 from Xcode)

Error

macro expansion @Observable:16:26: error: declaration name 'shouldNotifyObservers' is not covered by macro 'Observable'

The generated code shows mangled unique names:

private nonisolated func shouldNotifyObservers<$s6A_LIST22JourneysContentManager10ObservablefMm_6MemberfMu1_>(...)

Root Cause Analysis

The @Observable macro uses context.makeUniqueName("Member") for generic parameters. On Linux, these generate names starting with $ that look like Swift symbol manglings, which the compiler then rejects.

The macro declaration in Swift properly declares shouldNotifyObservers in its names: parameter:

@attached(member, names: named(_$observationRegistrar), named(access), named(withMutation), named(shouldNotifyObservers))

But the generated code uses unique mangled names that aren't being accepted.

Related Issues

• makeUniqueName() produces tokens that cannot be used as closure arguments swiftlang/swift-syntax#2256 (makeUniqueName produces $ prefixed names)
• Cross-compiling packages which contain macros doesn't work swiftlang/swift-package-manager#6950 (cross-compile macros)

Question

Is this a known limitation? Is there a workaround like using a different Swift version or toolchain configuration?

[kabiroberai]

Can you try the iOS 26 SDK? Macros for system libraries tend to be tightly coupled between the compiler version and the SDK. The iOS 18 SDK probably expects Swift 6.1, and iOS 26 expects 6.2.

[guitaripod]

Update: Found workaround - Swift 6.1 works

After testing, I can confirm that Swift 6.1 does not have this issue. The @Observable macro works correctly with Swift 6.1 on Linux.

This appears to be a regression in Swift 6.2.x.

However, after getting past this issue with Swift 6.1, I hit another blocker: the Sentry xcframework causes a signing error:

Invalid arch file in fat mach-o file!

This is tracked in #130. So even with the @observable fix, xtool builds fail if the project uses Sentry.

For now, projects using:

• @Observable + Swift 6.2.x → blocked by this issue
• Sentry xcframework → blocked by Issues using the Sentry package #130

I've also filed the underlying Swift bug: swiftlang/swift#86778

[amgdev9]

@guitaripod I tested on swift 6.2.4 and Observable macros work

[guitaripod]

Tested this extensively with a real-world app (~250 source files, 11 dependencies including Firebase, Supabase, Sentry, WebRTC).

Swift 6.2.4 does NOT fix @Observable with the iOS 18.5 SDK. The issue is the SDK, not the compiler version.

Root cause

The iOS 18.5 SDK's Observation.swiftmodule swiftinterface was compiled with Swift 6.1.2. The @Observable macro declaration allows these member names:

names: named(_$observationRegistrar), named(access), named(withMutation)

Swift 6.2.x's ObservationMacros plugin generates an additional shouldNotifyObservers member, which the SDK's declaration doesn't permit → compile error.

Even after patching the swiftinterface to add named(shouldNotifyObservers) (which fixes compilation), linking fails with:

ld64.lld: error: undefined symbol: swift_coroFrameAlloc

swift_coroFrameAlloc is a new Swift 6.2 runtime function that doesn't exist in the iOS 18.5 SDK's runtime libraries.

What actually works

Swift 6.1.2 matches the iOS 18.5 SDK perfectly — both compilation and linking succeed. The SDK was built with Swift 6.1.2, so the compiler/SDK pair is fully compatible.

To use Swift 6.2.x

You'd need an iOS 26 SDK (from Xcode 26) which includes:

• Updated @Observable macro declaration with shouldNotifyObservers
• Swift 6.2 runtime with swift_coroFrameAlloc

Other notes for anyone trying to build a real app with xtool

• #Preview macros fail on Linux (PreviewsMacros plugin not available) — wrap with conditional compilation
• swift-tools-version: 6.0 enables strict concurrency by default — use .swiftLanguageMode(.v5) in target settings unless your codebase is Swift 6 ready
• Swift regex literals (/.../) may fail to parse during cross-compilation — use NSRegularExpression as fallback
• -ObjC linker flag is not supported by ld64.lld on Linux

[guitaripod]

Update: iOS 26.2 SDK (from Xcode 26.2) + Swift 6.2.4 works cleanly.

Followed up on my earlier comment — switching from iOS 18.5 SDK to iOS 26.2 SDK eliminates nearly every workaround:

Issue	iOS 18.5 SDK	iOS 26.2 SDK
@Observable / shouldNotifyObservers	Broken (SDK swiftinterface missing member)	Works natively
swift_coroFrameAlloc linker error	Broken (runtime symbol missing)	Works natively
.allowBluetoothHFP	Missing from SDK	Present in SDK
#Preview macros	Broken (no PreviewsMacros plugin on Linux)	Still broken — platform limitation
Swift regex literals (/.../)	Broken during cross-compilation	Still broken — compiler bug

Only 2 source-level changes needed with iOS 26.2 SDK:

1. #Preview blocks → wrap with #if !XTOOL_BUILD (113 files in our case, scriptable)
2. Regex literals → use Regex(#"pattern"#) string constructor instead of /.../ literals (2 files). Same modern Swift Regex API, just avoids the literal parser bug. NSRegularExpression also works but is unnecessary — the bug is only in the literal syntax, not the Regex type.

Note on Xcode versions: Xcode 26.3's SDK ships swiftlang-6.2.3.3.2 which uses ~Copyable suppression syntax not yet in open-source Swift 6.2.4 — use Xcode 26.2 instead.

Build: xtool dev build completes in ~220s for a ~250-file app with 11 dependencies (Firebase, Supabase, Sentry, WebRTC, etc).

[guitaripod]

Consider merging this. It works.

• [Fix Sentry package bugs: name mismatch and FAT static detection #198]