SILGen crash in emitPackExpansionIntoPack when calling cross-module function with pack expansion argument

[coenttb]

Description

The Swift compiler crashes with signal 11 during SIL generation when calling a function from another module that takes a parameter pack expansion (repeat each T) as an argument.

The same code works correctly when the function is defined in the same module.

Environment

• Swift version: 6.2.3 (swiftlang-6.2.3.3.21 clang-1700.6.3.2)
• Target: arm64-apple-macosx26.0
• Crash location: emitPackExpansionIntoPack in SILGenApply.cpp

Minimal Reproduction

https://github.com/coenttb/swift-issue-silgen-pack-expansion-cross-module

LibraryA (defines the function):

public enum Tuple {
    public enum Append {}
}

extension Tuple.Append {
    @inlinable
    public static func apply<each T, U>(
        _ tuple: (repeat each T),
        _ element: U
    ) -> (repeat each T, U) {
        (repeat each tuple, element)
    }
}

LibraryB (calls the function — CRASHES):

public import LibraryA

@inlinable
public func combine<each O1, O2>(
    _ first: (repeat each O1),
    _ second: O2
) -> (repeat each O1, O2) {
    Tuple.Append.apply(first, second)  // ❌ CRASHES
}

To Reproduce

git clone https://github.com/coenttb/swift-issue-silgen-pack-expansion-cross-module
cd swift-issue-silgen-pack-expansion-cross-module
swift build

Crash Output

error: compile command failed due to signal 11 (use -v to see invocation)
Stack dump:
0.  Program arguments: swift-frontend -frontend -emit-module ...
1.  Apple Swift version 6.2.3 (swiftlang-6.2.3.3.21 clang-1700.6.3.2)
2.  Compiling with the current language version
3.  While evaluating request ASTLoweringRequest(Lowering AST to SIL for module LibraryB)
4.  While silgen emitFunction SIL function "@$s8LibraryB7combine..."
    for 'combine(_:_:)' (at .../Caller.swift:21:1)

4  swift-frontend  emitPackExpansionIntoPack(...) + 492
5  swift-frontend  emitPackExpansionIntoPack(...) + 492
6  swift-frontend  emitDynamicPackLoop(...) + 1904
7  swift-frontend  ArgEmitter::emitPackArg(...) + 3532
...

Key Stack Frames

Frame	Function	File
4-5	emitPackExpansionIntoPack	SILGenApply.cpp
6	emitDynamicPackLoop	SILGenPack.cpp
7	ArgEmitter::emitPackArg	SILGenApply.cpp

Conditions Required

All three conditions must be present to trigger the crash:

Condition	Description
1. Cross-module call	Function defined in module A, called from module B
2. Pack expansion argument	Passing (repeat each T) as function argument
3. @inlinable	Function must be inlinable (for cross-module inlining)

Verified Test Results

Test	Description	Result
Same module	Function and caller in same module	✅ Compiles
Cross-module	Function in LibraryA, caller in LibraryB	❌ Crashes
Inline expression	(repeat each tuple, element) directly	✅ Compiles
Function call	Tuple.Append.apply(tuple, element)	❌ Crashes

Workaround

Inline the pack expansion expression instead of calling a cross-module function:

// Instead of:
Tuple.Append.apply(tuple, element)  // ❌ Crashes

// Use:
(repeat each tuple, element)  // ✅ Works

This workaround prevents code reuse across modules for pack expansion operations.

Impact

This bug blocks:

• Creating reusable tuple/product utilities in shared libraries
• Result builder implementations that want to share pack manipulation code
• Any cross-module abstraction over parameter pack operations

Possibly Related Issues

• Crash in visitPackExpansionExpr, file SILGenExpr.cpp, line 1655 when writing variadically generic zip function #84568 - Crash in visitPackExpansionExpr
• Crash at compilation when calling variadic generic function using pack expansion as argument #67645 - Crash calling variadic generic function with pack expansion
• Parameter Pack Crashes within Generic Type #76391 - Parameter pack crashes within generic type