Adding a conformance apparently corrupts metadata cache?

[dabrahams]

Description
If I add a conformance to my project—one that isn't even used—I get a crash during testing.

Steps to reproduce
Check out the swift-conformance-crasher tag of the Val project. swift test --filter testEmitter. For me, this crashes (see attached screenshot).
Now delete the conformance of WideUInt to BinaryInteger and run the test again. For me, the test passes.

Furthermore, I can add

extension WideUInt: UnsignedInteger {}

to another module (e.g. Compiler) without incident.

Of course this doesn't prove the bug is in the compiler/runtime, but it's highly suspicious.

Expected behavior
The presence of that conformance should have no effect.

Environment

• Swift compiler version info swift-driver version: 1.62.15 Apple Swift version 5.7.1 (swiftlang-5.7.1.135.3 clang-1400.0.29.51)
  Target: arm64-apple-macosx13.0
• Xcode version info Xcode 14.1
  Build version 14B47b
• Deployment target: not explicitly set.

[dabrahams]

I'm still reproducing this in the hylo project, FWIW.

[mikeash]

I wasn't able to reproduce this locally, but from your screenshot it looks like you're somehow encountering infinite recursion. It just so happens that the MetadataCache code is what runs off the end of the stack and crashes. Seems that adding that conformance somehow makes part of this code call back into itself, where it doesn't do so otherwise.

[dabrahams]

@mikeash, I'm really surprised it doesn't reproduce for you, as I've seen it on multiple machines. Did you try with the Hylo project, moving the WideUInt: UnsignedInteger conformance from Sources/IR/Operands/Constant/IntegerConstant.swift to Sources/Utils/WideUInt.swift?

[mikeash]

I tried it with the swift-conformance-crasher tag of both Hylo and Val. I'm using Xcode 15.0.1, not sure if that matters. When I try to build Hylo main, it fails to build due to not finding the types SPMBuildCommand and SPMBuildToolPlugin.

[dabrahams]

You need to `git submodule update --init`. Note also, Val became Hylo, so when I asked did you try with Hylo, I just meant have you tried with the HEAD of main. BUT: I just got xcode 15.0.1 and it's not reproducing on this machine, so maybe the issue can be closed.

[dabrahams]

@mikeash Even with xcode 15.0.1, swift --version still says 5.9, and swift package --version says 5.9.0. I'm running this build using SPM, so I don't see how xcode can come into the picture. How am I supposed to tell SPM that this requires xcode 15.0.1?

[mikeash]

swift will use whatever Xcode you have selected, or the Command Line Tools if you have those installed. You can run xcode-select -p to see what's selected, and xcode-select -s /path/to/copy/of/Xcode.app to change it. You can also change it temporarily by setting the DEVELOPER_DIR environment variable to the path you want to use locally.

If the issue doesn't happen with 15.0.1 then I'd guess there's some change in method or function resolution which previously resulted in choosing a function that produced recursion, and now it doesn't. Hard to say if it was a bug fix or a just a change, but either way, it's good that it works now.

[dabrahams]

I think you're misunderstanding my question. If I am going to put the conformance back where it belongs I need to use a swift-tools-version spec in Package.swift to declare that Xcode 15.0.1 is required, right? How can I do that if the Swift version didn't change between Xcode 15.0 and 15.0.1?

[mikeash]

Oh, I see. Does it break with 15 but work with 15.0.1? That would be really weird. I don’t think there were any changes to the compiler.