Build error with macOS OSS toolchain: cannot find type 'MapContent' in scope

[marcprux]

Description

Trying to cross-compile to the static Linux SDK from macOS fails with the error:

% ~/Library/Developer/Toolchains/swift-6.1-RELEASE.xctoolchain/usr/bin/swift build --swift-sdk x86_64-swift-linux-musl

[1/1] Planning build
Building for debugging...
/private/tmp/swift-perception/Sources/PerceptionCore/SwiftUI/WithPerceptionTracking.swift:227:39: error: cannot find type 'MapContent' in scope
225 | 
226 |     @available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
227 |     extension WithPerceptionTracking: MapContent where Content: MapContent {
    |                                       `- error: cannot find type 'MapContent' in scope
228 |       public init(@MapContentBuilder content: @escaping () -> Content) {
229 |         self.init(content: content())

/private/tmp/swift-perception/Sources/PerceptionCore/SwiftUI/WithPerceptionTracking.swift:227:65: error: cannot find type 'MapContent' in scope
225 | 
226 |     @available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
227 |     extension WithPerceptionTracking: MapContent where Content: MapContent {
    |                                                                 `- error: cannot find type 'MapContent' in scope
228 |       public init(@MapContentBuilder content: @escaping () -> Content) {
229 |         self.init(content: content())

/private/tmp/swift-perception/Sources/PerceptionCore/SwiftUI/WithPerceptionTracking.swift:228:20: error: unknown attribute 'MapContentBuilder'
226 |     @available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
227 |     extension WithPerceptionTracking: MapContent where Content: MapContent {
228 |       public init(@MapContentBuilder content: @escaping () -> Content) {
    |                    `- error: unknown attribute 'MapContentBuilder'
229 |         self.init(content: content())
230 |       }
error: emit-module command failed with exit code 1 (use -v to see invocation)
/private/tmp/swift-perception/Sources/PerceptionCore/SwiftUI/WithPerceptionTracking.swift:227:39: error: cannot find type 'MapContent' in scope
225 | 
226 |     @available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
227 |     extension WithPerceptionTracking: MapContent where Content: MapContent {
    |                                       `- error: cannot find type 'MapContent' in scope
228 |       public init(@MapContentBuilder content: @escaping () -> Content) {
229 |         self.init(content: content())

/private/tmp/swift-perception/Sources/PerceptionCore/SwiftUI/WithPerceptionTracking.swift:227:65: error: cannot find type 'MapContent' in scope
225 | 
226 |     @available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
227 |     extension WithPerceptionTracking: MapContent where Content: MapContent {
    |                                                                 `- error: cannot find type 'MapContent' in scope
228 |       public init(@MapContentBuilder content: @escaping () -> Content) {
229 |         self.init(content: content())

/private/tmp/swift-perception/Sources/PerceptionCore/SwiftUI/WithPerceptionTracking.swift:228:20: error: unknown attribute 'MapContentBuilder'
226 |     @available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
227 |     extension WithPerceptionTracking: MapContent where Content: MapContent {
228 |       public init(@MapContentBuilder content: @escaping () -> Content) {
    |                    `- error: unknown attribute 'MapContentBuilder'
229 |         self.init(content: content())
230 |       }

This is pretty kooky, because WithPerceptionTracking.swift gates those lines within both #if canImport(SwiftUI) as well as #if canImport(MapKit), neither of which are available in the static Linux SDK.

swift-perception/Sources/PerceptionCore/SwiftUI/WithPerceptionTracking.swift

Lines 223 to 232 in 7d3509c

	#if canImport(MapKit)
	import MapKit
	@available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
	extension WithPerceptionTracking: MapContent where Content: MapContent {
	public init(@MapContentBuilder content: @escaping () -> Content) {
	self.init(content: content())
	}
	}
	#endif

Even weirder, if I comment out that section, the project builds fine, even though there is a similar construct with Charts that doesn't seem to cause any issues:

swift-perception/Sources/PerceptionCore/SwiftUI/WithPerceptionTracking.swift

Lines 214 to 220 in 7d3509c

	@available(iOS 16, macOS 13, tvOS 16, watchOS 9, *)
	extension WithPerceptionTracking: ChartContent where Content: ChartContent {
	@_disfavoredOverload
	public init(@ChartContentBuilder content: @escaping () -> Content) {
	self.init(content: content())
	}
	}

I suspect this must somehow be a bug in the Swift toolchain, but I can't narrow it down to a small reproducible example, so I figured I would report it here so you are aware of it.

FTR, this also affects the Android SDK, so any package that depends on swift-perception cannot currently be built for Android due to this issue.

Checklist

• I have determined that this bug is not reproducible using Swift's observation tools. If the bug is reproducible using the @Observable macro or another tool from the Observation framework, please file it directly with Apple.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

No response

Actual behavior

No response

Steps to reproduce

No response

Perception version information

7d3509c

Destination operating system

No response

Xcode version information

Command line SwiftPM

Swift Compiler version information

% ~/Library/Developer/Toolchains/swift-6.1-RELEASE.xctoolchain/usr/bin/swift --version
Apple Swift version 6.1 (swift-6.1-RELEASE)
Target: arm64-apple-macosx15.0

[mbrandonw]

Hi @marcprux, I'm just not sure there is much we can do here. We are taking all the precautions necessary to make sure this code does not compile on platforms that don't support it. Have you tried reporting this to Apple?

Since this isn't an issue with the library I am going to convert it to a discussion. Please feel free to continue the conversation over there!

[marcprux]

Agreed that I'm not sure if there's anything you can do, but I figured I'd document it in case anyone runs into the error when attempting to build.

[marcprux]

It turns out this doesn't have anything to do with the cross-compilation SDK: building anything with the macOS OSS toolchain (rather than Xcode's) will cause the build error, with or without a custom SDK. I.e.:

~/Library/Developer/Toolchains/swift-6.1-RELEASE.xctoolchain/usr/bin/swift build

/private/tmp/swift-perception/Sources/PerceptionCore/SwiftUI/WithPerceptionTracking.swift:227:39: error: cannot find type 'MapContent' in scope
225 | 
226 |     @available(iOS 17, macOS 14, tvOS 17, watchOS 10, *)
227 |     extension WithPerceptionTracking: MapContent where Content: MapContent {
    |                                       `- error: cannot find type 'MapContent' in scope
228 |       public init(@MapContentBuilder content: @escaping () -> Content) {
229 |         self.init(content: content())

[stephencelis]

@marcprux In another issue @tonyarnold mentioned looking into a fix for these imports: #151 (comment)

[marcprux]

Interesting, that does sound like a similar phenomenon. I wonder if this swift-perception maybe shouldn't include built-in adaptations for MapKit and Charts and other such specialized frameworks, and instead include the sample code as documentation for how a project might include these conformances themselves if they want it.

[stephencelis]

@marcprux We're hoping that Tony's fix can allow us to keep those conformances. It'd be a shame to require manual implementations due to a compiler bug.

[stephencelis]

@marcprux In the process we noticed that this extension was nonsensical, so we're removing it: #158

Does this PR branch build cleanly for Android?

[marcprux]

Yes, it builds fine, both for Android as well as the static linux SDK.