Use variadic generics and opaque types together

[GeorgeElsham]

Description
The compiler crashes when combining the use of variadic generics with opaque types.

Compiler crash message in Xcode:

Command SwiftEmitModule failed with a nonzero exit code

Steps to reproduce
Minimal reproducible example code:

protocol P {
    associatedtype New: P
    func new() -> New
}

struct A: P {
    func new() -> some P {
        A()
    }
}

struct B<each T: P>: P {
    func new() -> some P { // <- CHANGE ME
        B<A>()
    }
}

The above code fails to compile. Changing the second some P to the more explicit B<A> in this example fixes the issue.

Expected behavior
Successful compilation by having the return type be inferred with the opaque type rather than having to manually specify it.

Environment

• Swift compiler version info

  swift-driver version: 1.87.1 Apple Swift version 5.9 (swiftlang-5.9.0.128.106 clang-1500.0.40.1)
  Target: arm64-apple-macosx13.0

• Xcode version info

  Xcode 15.0
  Build version 15A5229m

• Deployment target

  iOS 17

[ole]

@GeorgeElsham Your code compiles successfully for me with Xcode 15.3b1 (Swift 5.10). Could you verify?

Environment:

$ sw_vers
ProductName:		macOS
ProductVersion:		14.3
BuildVersion:		23D56

$ swift --version
Apple Swift version 5.10 (swiftlang-5.10.0.10.5 clang-1500.3.7.4)
Target: arm64-apple-macosx14.0

$ xcodebuild -version
Xcode 15.3
Build version 15E5178i

[ole]

Actually, it even compiles with Xcode 15.2 (Swift 5.9.2).

[GeorgeElsham]

I updated to Xcode 15.3 Beta (15E5178i), and Xcode still gives me:

Command SwiftCompile failed with a nonzero exit code

Command SwiftEmitModule failed with a nonzero exit code

[ole]

Hm. Here's what I'm doing:

# This is your code, plus a print statement to verify that the program runs
$ cat swift-issue-68324.swift
protocol P {
    associatedtype New: P
    func new() -> New
}

struct A: P {
    func new() -> some P {
        A()
    }
}

struct B<each T: P>: P {
    func new() -> some P { // <- CHANGE ME
        B<A>()
    }
}

print(B<A, A>().new())

# Xcode 15.3b1
$ swift --version
swift-driver version: 1.90.8 Apple Swift version 5.10 (swiftlang-5.10.0.10.5 clang-1500.3.7.4)
Target: arm64-apple-macosx14.0

$ swiftc swift-issue-68324.swift

$ ./swift-issue-68324
B<Pack{A}>()

$ sudo xcode-select --switch /Applications/Xcode-15.2.app

$ swift --version
swift-driver version: 1.87.3 Apple Swift version 5.9.2 (swiftlang-5.9.2.2.56 clang-1500.1.0.2.5)
Target: arm64-apple-macosx14.0

$ swiftc swift-issue-68324.swift

$ ./swift-issue-68324
B<Pack{A}>()

[GeorgeElsham]

Bit frustrating. I indeed get the same as you when compiling manually with swiftc, it appears to work fine. However, the moment I add it to any file in any Swift package, Xcode gives these errors. Is it similar for you, if you just add this to a file in any Swift package?

[ole]

Yes, I can confirm. And it doesn't seem to be an Xcode issue. It also fails when invoking SwiftPM from the command line. Tested here with a recent nighly toolchain, but I also tested with Swift 5.10 (Xcode 15.3b1):

swift run --toolchain /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2024-01-30-a.xctoolchain/
Building for debugging...
error: emit-module command failed due to signal 4 (use -v to see invocation)
error: compile command failed due to signal 4 (use -v to see invocation)