Glibc import fails to cross-compile with Swift SDK when .Cxx interop is enabled

[xtremekforever]

Description

If I have a Swift package that has an import Glibc (either explicit or implicit via other imports) and try to cross compile it with a swift-DEVELOPMENT-SNAPSHOT-2025-08-25-a Swift SDK, I get the following errors:

error: emit-module command failed with exit code 1 (use -v to see invocation)
~/Library/org.swift.swiftpm/swift-sdks/DEVELOPMENT-SNAPSHOT-2025-08-25-a_debian_bookworm_aarch64.artifactbundle/DEVELOPMENT-SNAPSHOT-2025-08-25-a_debian_bookworm_aarch64/aarch64-unknown-linux-gnu/debian-bookworm.sdk/usr/lib/swift/linux/Glibc.swiftmodule/aarch64-unknown-linux-gnu.private.swiftinterface:92:44: error: cannot use type alias '__sighandler_t' here; C++ types from imported module 'SwiftGlibc' do not support library evolution
 90 | public func ioctl(_ fd: Swift.CInt, _ request: Swift.UInt, _ ptr: Swift.UnsafeMutableRawPointer) -> Swift.CInt
 91 | public func ioctl(_ fd: Swift.CInt, _ request: Swift.UInt) -> Swift.CInt
 92 | public typealias sighandler_t = SwiftGlibc.__sighandler_t
    |                                            `- error: cannot use type alias '__sighandler_t' here; C++ types from imported module 'SwiftGlibc' do not support library evolution
 93 | public var SIG_DFL: Glibc.sighandler_t? {
 94 |   get

~/Library/org.swift.swiftpm/swift-sdks/DEVELOPMENT-SNAPSHOT-2025-08-25-a_debian_bookworm_aarch64.artifactbundle/DEVELOPMENT-SNAPSHOT-2025-08-25-a_debian_bookworm_aarch64/aarch64-unknown-linux-gnu/debian-bookworm.sdk/usr/include/signal.h:72:16: note: type alias declared here
 70 |
 71 | /* Type of a signal handler.  */
 72 | typedef void (*__sighandler_t) (int);
    |                `- note: type alias declared here
 73 |
 74 | /* The X/Open definition of `signal' specifies the SVID semantic.  Use

~/Programming/tmp/swift-hello/Sources/swift-hello/swift_hello.swift:4:8: error: failed to build module 'Glibc'; this SDK is not supported by the compiler (the SDK is built with 'Swift version 6.2-dev effective-5.10 (LLVM 54eee003e08e203, Swift db8b8480e97e37f)', while this compiler is 'Apple Swift version 6.2-dev effective-5.10 (LLVM 54eee003e08e203, Swift db8b8480e97e37f)'). Please select a toolchain which matches the SDK.
2 | // https://docs.swift.org/swift-book
3 |
4 | import Glibc
  |        `- error: failed to build module 'Glibc'; this SDK is not supported by the compiler (the SDK is built with 'Swift version 6.2-dev effective-5.10 (LLVM 54eee003e08e203, Swift db8b8480e97e37f)', while this compiler is 'Apple Swift version 6.2-dev effective-5.10 (LLVM 54eee003e08e203, Swift db8b8480e97e37f)'). Please select a toolchain which matches the SDK.
5 |
6 | print("Hello, World!")

There appears to be some sort of bug in Glibc that causes it to fail cross-compilation when .Cxx interop is enabled. @MaxDesiatov

Reproduction

1. Create a swift-hello package with the following contents:

import Glibc

print("Hello, World!")

2. Create a Swift SDK using the swift-sdk-generator: swift run swift-sdk-generator make-linux-sdk --swift-branch development --swift-version DEVELOPMENT-SNAPSHOT-2025-08-25-a --target aarch64-unknown-linux-gnu --distribution-name debian --distribution-version 12
3. Install the Swift SDK: swift sdk install Bundles/DEVELOPMENT-SNAPSHOT-2025-08-25-a_debian_bookworm_aarch64.artifactbundle
4. Compile the swift-hello package using the Swift SDK and passing the -cxx-interoperability-mode=default flag to swiftc: swift build --swift-sdk DEVELOPMENT-SNAPSHOT-2025-08-25-a_debian_bookworm_aarch64 -Xswiftc -cxx-interoperability-mode=default

Expected behavior

The exact same setup works fine with Swift versions from 5.9 all the way up to 6.1.2. So, I'd expect that cross-compiling with .Cxx interop in 6.2 and main would also work instead of throwing library evolution errors when Glibc is imported.

Environment

I get the same errors cross-compiling with the Swift SDK on macOS and Linux. Here are my environments:

macOS
Apple Swift version 6.2-dev (LLVM 54eee003e08e203, Swift db8b848)
Target: arm64-apple-macosx26.0
Build config: +assertions
Darwin Yang.local 25.0.0 Darwin Kernel Version 25.0.0: Thu Aug 14 21:59:19 PDT 2025; root:xnu-12377.1.8~5/RELEASE_ARM64_T6000 arm64

Linux
Swift version 6.2-dev (LLVM 54eee003e08e203, Swift db8b848)
Target: x86_64-unknown-linux-gnu
Build config: +assertions
Linux ella 6.14.0-27-generic #27~24.04.1-Ubuntu SMP PREEMPT_DYNAMIC Tue Jul 22 17:38:49 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

Additional information

This seems to also happen with much older 6.2 and main snapshots from several months ago as well. I will note, however, that I have a (proprietary) Swift package that enables C++ interop but mysteriously cross-compiles fine with the 6.2 and main snapshots. However, my package has lots of included C++11 code inside of it, so maybe that changes the behavior of compilation- or at least that is the theory.

[xtremekforever]

I went into main to find out where things broke here and traced it down to the swift-DEVELOPMENT-SNAPSHOT-2025-02-25-a. The previous snapshot swift-DEVELOPMENT-SNAPSHOT-2025-02-24-a worked perfectly fine, then the build from Feb 25th started showing the issue. So, that narrows it down to one of the commits here on Feb 25, 2025:

https://github.com/swiftlang/swift/commits/swift-DEVELOPMENT-SNAPSHOT-2025-02-25-a

On the 6.2 branch I found that this regression made it in somewhere between swift-6.2-DEVELOPMENT-SNAPSHOT-2025-04-21-a (first ever 6.2 snapshot) and swift-6.2-DEVELOPMENT-SNAPSHOT-2025-04-23-a. So presumably main got merged in or cherry picked and included the bug.

[finagolfin]

I couldn't reproduce with recent static linux SDK or Android snapshots, so this appears specific to glibc. Can you paste the output with the -v flag added, for both the failing April 23 6.2 snapshot and the working April 21 6.2 snapshot? That might have useful info for @egorzhdan or other Swift devs to look at.

[xtremekforever]

@finagolfin omg I tried the 6.2 April 21st snapshot again and now it's failing as I had seen the first time I tried it. I'm thinking the 2nd time I tried I must have missed a flag or something. So, as it stands, we only have the main-snapshot 2025-02-24 that works, and 2025-02-25 starts showing this behavior.

I ran the build commands with -v with each snapshot and captured the output.

cross-compile-log-DEVELOPMENT-SNAPSHOT-2025-02-24-a.txt
cross-compile-log-DEVELOPMENT-SNAPSHOT-2025-02-25-a.txt

If there's anything I notice between the 2 logs above, it's that it appears that things get compiled in different orders after the first swiftc -module-name swift_hello -emit-dependencies -emit-module -emit-module-path invocation. I wonder if that would make a big difference...

[finagolfin]

I compared the flags, no difference, so probably a Swift compiler change. Nothing stands out for me in the git commits that day, either in the LLVM changes or in lib/ClangImporter/.

@hnrklssn or @Xazax-hun, any idea what might have caused this linux regression with C++ Interop?

[Xazax-hun]

Not sure why is this a regression. But could you try if #84040 fixes it?