Runtime Crash at "__swift_instantiateConcreteTypeFromMangledName"

[MahdiBM]

Description

DiscordBM crashes at __swift_instantiateConcreteTypeFromMangledName.

Reproduction

• Clone DiscordBM.
• On CLI run BOT_TOKEN=aaaaaa swift test --filter IntegrationTests.DiscordClientTests.testGateway.
• Or on Xcode add BOT_TOKEN as an env var with any value, and run the testGateway() test.

Stack dump

Expected behavior

No crash.

Environment

• macOS 14 + Swift 5.10 + Xcode 15.4.
• macOS 15 + Swift 6 + Xcode 16.

Not reproducible on Linux.

Additional information

No response

[MarkVillacampa]

I believe this could be the same issue: #74333

[MahdiBM]

I tried a bunch and i noticed if i comment out this line in PartialApplication declaration, it fixes the problem, at least partially (tried these on the be95b07e3cc9db075d201f14051589676dd99785 commit):

    public var flags: IntBitField<DiscordApplication.Flag>?

So IntBitField<DiscordApplication.Flag>? must be causing the problem.

• Using typealiases doesn't help.
• removing the optional sign doesn't help.
• Adding explicit Codable conformance doesn't help, but Xcode will show a line in the decoder as the source of the crash instead: self.flags = try container.decodeIfPresent(IntBitField<DiscordApplication.Flag>.self, forKey: PartialApplication.CodingKeys.flags)
• Doing stuff like let uint = try container.decodeIfPresent(UInt.self, forKey: PartialApplication.CodingKeys.flags); self.flags = uint.map { .init(rawValue: $0) } in the decoder does not help.

[MahdiBM]

This issue is still present in Xcode 16 beta 3.
I hope it gets resolved before the RC since a few users of DiscordBM have reported it to me 🙁.

[tbkka]

CC: @al45tair @mikeash

[mikeash]

Which commit should I be using with beta 3? I tried 7865b015f9135d8e1799a0f406be0174e4b04d85 (which is current ToT for main) and got a bunch of "error: pattern that the region based isolation checker does not understand how to check. Please file a bug." (Filed rdar://131471942 internally for that.) I tried ff804c560d14e5993261ad1bed923766c1b52265 which is just before mentions of Swift 6 and that crashes with a stack overflow. Then tried be95b07e3cc9db075d201f14051589676dd99785 and that seems to work fine. It's possible I need to more closely match the macOS build, but before I do that I want to make sure I'm targeting a commit that's known to build and crash (in the appropriate way!) with beta 3.

[MahdiBM]

I would also really appreciate it if there are any workarounds to this so I can apply so earlier Swift versions can also continue to work properly 🙂.

[MahdiBM]

Still an issue in Xcode 16 beta 4.

[MahdiBM]

Still an issue on Xcode 16 beta 2.
And still getting frequent user reports about this issue which I have 0 idea how to even work-around.

Pretty frustrating.

I don't even know if looking around in the Swift repo is any good considering this only happens on macOS and Xcode contains a fork of the Swift repo, not the repo itself :/
Not that I'm any good at compiler work, but could have tried.

cc @tbkka @mikeash @hborla