Weird issue with @DependencyClient and typed throws

[finestructure]

Happy New Year 🥳🙂

I'm encountering a baffling issue trying to add a dependency with a typed throwing function in a @DependencyClient. The client looks like this (trimmed down):

@DependencyClient
struct GithubClient2 {
    var fetchLicense: @Sendable (_ owner: String, _ repository: String) async -> Github2.License?
    var fetchMetadata: @Sendable (_ owner: String, _ repository: String) async throws(Github2.Error) -> Github2.Metadata = { _, _ in XCTFail("fetchMetadata"); return .init() }
}

The macro is failing to compile with an error inside the generated code

   +--- macro expansion @DependencyClient ------------------------------
   | 9 |
   |10 | init(
   |11 |     fetchMetadata: @Sendable (_ owner: String, _ repository: String) async throws (Github2.Error) -> Github2.Metadata = {
   |   |     `- note: parameter 'fetchMetadata' is implicitly non-escaping
   |12 |     _, _ in XCTFail("fetchMetadata"); return .init() }
   |13 | ) {
   |14 | self.fetchMetadata = fetchMetadata
   |   |                    `- error: assigning non-escaping parameter 'fetchMetadata' to an @escaping closure
   |15 | }
   +--------------------------------------------------------------------

I went to create a minimal reproducer for the issue - only to find that it doesn't reproduce in this new package. It compiles fine.

I then went and copied the exact same source file into the main project and it fails to compile with the same error as the original attempt.

I've compared the package manifests - both tools-version 6.0, same dependency versions - and I'm at a loss what the difference could possibly be.

As the title indicates, this seems to be related to the typed throw throws(Github2.Error). If I make the closure throws it compiles.

But not only that: it also compiles, even with the typed throw, if I comment out the fetchLicense closure 🤯:

@DependencyClient
struct GithubClient2 {
    // var fetchLicense: @Sendable (_ owner: String, _ repository: String) async -> Github2.License?
    var fetchMetadata: @Sendable (_ owner: String, _ repository: String) async throws(Github2.Error) -> Github2.Metadata = { _, _ in XCTFail("fetchMetadata"); return .init() }
}

So to summarise:

	dep_test project	SPI Server project
throws(Github2.Error), two closures	✅	❌
throws, two closures	✅	✅
throws(Github2.Error), single closure	✅	✅

Any ideas what might be going on? Why would this fail embedded in the bigger project while it works for all scenarios in the standalone one? Any ideas what I could look at in the main project to further isolate the issue?

Just to make sure I'm not imagining things, I've re-run everything with fresh checkouts. Here's the recipe to reproduce:

git clone https://github.com/finestructure/dep-test  # clone "repro" project
cd dep-test/ && swift build   # demonstrate no build error in dep-test
cd ..
git clone https://github.com/SwiftPackageIndex/SwiftPackageIndex-Server --depth 1 -b 2.115.2  # clone main project at tag
cp dep-test/Sources/dep-test/dep_test.swift SwiftPackageIndex-Server/Sources/App/  # copy source into main project
cd SwiftPackageIndex-Server/ && swift build  # try and build main project

And my transcript:

❯ swift --version
swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx15.0
~/Downloads
❯ xcodebuild -version
Xcode 16.2
Build version 16C5032a
❯ git clone https://github.com/finestructure/dep-test  # clone "repro" project
... <- output truncated -> ...
~/Downloads
❯ cd dep-test/ && swift build   # demonstrate no build error in dep-test
... <- output truncated -> ...
Building for debugging...
[346/346] Compiling dep_test dep_test.swift
Build complete! (30.23s)
~/D/dep-test on main took 30s
❯ cd ..
~/Downloads
✦ ❯ git clone https://github.com/SwiftPackageIndex/SwiftPackageIndex-Server --depth 1 -b 2.115.2  # clone main project at tag
... <- output truncated -> ...
~/Downloads took 9s
❯ cp dep-test/Sources/dep-test/dep_test.swift SwiftPackageIndex-Server/Sources/App/  # copy source into main project
~/Downloads
❯ cd SwiftPackageIndex-Server/ && swift build  # try and build main project
... <- output truncated -> ...
macro expansion @DependencyClient:14:20: error: assigning non-escaping parameter 'fetchMetadata' to an @escaping closure
`- /Users/sas/Downloads/SwiftPackageIndex-Server/Sources/App/dep_test.swift:9:1: note: expanded code originates here
 7 |     var fetchLicense: @Sendable (_ owner: String, _ repository: String) async -> Github2.License?
 8 |     var fetchMetadata: @Sendable (_ owner: String, _ repository: String) async throws(Github2.Error) -> Github2.Metadata = { _, _ in XCTFail("fetchMetadata"); return .init() }
 9 | }
   +--- macro expansion @DependencyClient ------------------------------
   | 9 |
   |10 | init(
   |11 |     fetchMetadata: @Sendable (_ owner: String, _ repository: String) async throws (Github2.Error) -> Github2.Metadata = {
   |   |     `- note: parameter 'fetchMetadata' is implicitly non-escaping
   |12 |     _, _ in XCTFail("fetchMetadata"); return .init() }
   |13 | ) {
   |14 | self.fetchMetadata = fetchMetadata
   |   |                    `- error: assigning non-escaping parameter 'fetchMetadata' to an @escaping closure
   |15 | }
   +--------------------------------------------------------------------
10 |
11 |
[4951/4954] Compiling App AuthorController.swift
~/D/SwiftPackageIndex-Server on HEAD (17ee055) took 1m37s

[finestructure]

FWIW, I was able to work around this by simply not using the @DependencyClient macro for this particular dependency client struct. So the issue seems to be clearly related to the macro expansion, or at least how the macro expansion works in our project.

[stephencelis]

@finestructure I think the library still needs to be updated to support typed throws. It hasn't been on the top of our list but if anyone tackles a PR we will gladly merge it!

[finestructure]

But it all seems to be working fine in the standalone project 🤔 Wouldn't you expect it to fail to compile there as well then?

[mbrandonw]

I'm not sure why it doesn't work in your project, but the minimal project you provided does work the way I expect. Since you are providing a default closure for fetchMetadata, the typed throws aspect doesn't even come into play. The only reason @DependencyClient doesn't work with type throws is because if you don't provide a default the macro will provide one for you that simply throws an error. But with typed throws we have no way of constructing that error.

And looking more closely at your error:

   +--- macro expansion @DependencyClient ------------------------------
   | 9 |
   |10 | init(
   |11 |     fetchMetadata: @Sendable (_ owner: String, _ repository: String) async throws (Github2.Error) -> Github2.Metadata = {
   |   |     `- note: parameter 'fetchMetadata' is implicitly non-escaping
   |12 |     _, _ in XCTFail("fetchMetadata"); return .init() }
   |13 | ) {
   |14 | self.fetchMetadata = fetchMetadata
   |   |                    `- error: assigning non-escaping parameter 'fetchMetadata' to an @escaping closure
   |15 | }
   +--------------------------------------------------------------------

…it seems that the real problem is that for some the initializer generated by @DependencyClient isn't using @escaping on the argument. It's also weird that the fetchLicense argument isn't in the initializer either.

So, while it may seem related to typed throws, I think that's a bit of a red herring. It just seems that something about the syntax of these endpoints is causing the macro to create an invalid initializer. But it's hard to investigate without a repro unfortunately.