Variadic-generic opaque return types produce malformed module interfaces

[karwa]

Description

The reproduction case compiles just fine if you try a normal build:

> swiftc variadic_opaque.swift -O
>

Boring, I know. No warnings, errors, nothing. Now let's enable library evolution:

> swiftc variadic_opaque.swift -O -enable-library-evolution
>

Also uneventful. Okay, now let's emit the module interface (we need to be in library evolution mode for this):

> swiftc variadic_opaque.swift -O -enable-library-evolution -emit-module-interface-path ./some_path
>
error: verify-emitted-module-interface command failed with exit code 1 (use -v to see invocation)
variadic_opaque_interface:13:104: error: pack expansion 'repeat each T' can only appear in a function parameter list, tuple element, or generic argument of a variadic type
11 |   
12 |   public typealias Element = Swift.Int
13 |   public typealias Iterator = @_opaqueReturnTypeOf("$s15variadic_opaque3FooV12makeIteratorQryF", 0) __<repeat each T>
   |                                                                                                        `- error: pack expansion 'repeat each T' can only appear in a function parameter list, tuple element, or generic argument of a variadic type
14 | }
15 | 

variadic_opaque_interface:9:15: error: type 'Foo<repeat each T>' does not conform to protocol 'Sequence'
 7 | import _StringProcessing
 8 | import _SwiftConcurrencyShims
 9 | public struct Foo<each T> : Swift.Sequence {
   |               |- error: type 'Foo<repeat each T>' does not conform to protocol 'Sequence'
   |               `- note: add stubs for conformance
10 |   public func makeIterator() -> some Swift.IteratorProtocol<Swift.Int>
11 |   

Swift.Sequence.Iterator:2:16: note: protocol requires nested type 'Iterator'
1 | protocol Sequence {
2 | associatedtype Iterator : IteratorProtocol}
  |                `- note: protocol requires nested type 'Iterator'
3 | 

variadic_opaque_interface:10:15: error: expected '{' in body of function declaration
 8 | import _SwiftConcurrencyShims
 9 | public struct Foo<each T> : Swift.Sequence {
10 |   public func makeIterator() -> some Swift.IteratorProtocol<Swift.Int>
   |               `- error: expected '{' in body of function declaration
11 |   
12 |   public typealias Element = Swift.Int

variadic_opaque_interface:1:1: error: failed to verify module interface of 'variadic_opaque' due to the errors above; the textual interface may be broken by project issues or a compiler bug
 1 | // swift-interface-format-version: 1.0
   | `- error: failed to verify module interface of 'variadic_opaque' due to the errors above; the textual interface may be broken by project issues or a compiler bug
 2 | // swift-compiler-version: Apple Swift version 6.1 effective-5.10 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
 3 | // swift-module-flags: -target arm64-apple-macosx15.0 -enable-objc-interop -enable-library-evolution -O -module-name variadic_opaque

Seems like it's doing something weird with the iterator typealias, leaving a dangling <repeat each T> on the end:

public typealias Iterator = @_opaqueReturnTypeOf("$s15variadic_opaque3FooV12makeIteratorQryF", 0) __<repeat each T>

When building an XCFramework including a different type that exposed this bug, I found I was able to entirely build Xcode archives of my projects and package them in to a framework without this module verifier running (doesn't seem to be on by default, it seems), but clients were unable to import the resulting framework.

Reproduction

public struct Foo<each T>: Sequence {
    var pack: (repeat each T)

    public func makeIterator() -> some IteratorProtocol<Int> {
        MyIter(pack: (repeat each pack))
    }
}

internal struct MyIter<each T>: IteratorProtocol {
    var pack: (repeat each T)
    mutating func next() -> Int? { 0 }
}

Expected behavior

Expect this code to produce a valid module interface that can be read by other compilers.

Environment

swift-driver version: 1.120.5 Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
Target: arm64-apple-macosx15.0

Additional information

No response