`compactMap { $0 as? Protocol }` on `[AnyObject?]` causes SIGSEGV under `-O`, works under `-Onone`

[Julioacarrettoni]

Description

When an array is typed as [AnyObject?] and elements are filtered and downcast via compactMap { $0 as? SomeProtocol }, the program crashes with a segmentation fault (exit code 139) in optimized builds. The exact same code runs correctly in debug builds. Changing the array type to [SomeProtocol?] and using compactMap { $0 } eliminates the crash.

This was first encountered in production code following this pattern:

Full code also available here: SwiftAnyObjectCast.zip

let items: [AnyObject?] = [
    ConcreteSection(title: "Always"),
    condition ? ConcreteSection(title: "Optional") : nil,
]
self.sections = items.compactMap { $0 as? SettingsSection }

Reproduction

protocol SettingsSection: AnyObject {
    var title: String { get }
    func process()
}

final class ConcreteSettingsSection: SettingsSection {
    let title: String
    init(title: String) { self.title = title }
    func process() { _ = title.uppercased() }
}

@inline(never)
func buildSections(includeOptional: Bool) -> [SettingsSection] {
    let optionalSection = ConcreteSettingsSection(title: "Optional Section")
    let optionalSections: [AnyObject?] = [
        ConcreteSettingsSection(title: "Non Optional Section"),
        includeOptional ? optionalSection : nil,
    ]
    return optionalSections.compactMap { $0 as? SettingsSection }
}

@inline(never)
func consume(_ sections: [SettingsSection]) {
    for section in sections {
        section.process()
        _ = section.title
    }
}

for i in 0..<10_000 {
    consume(buildSections(includeOptional: i.isMultiple(of: 2)))
}
print("Done")

Steps to reproduce

1. Use the reproduction code here or create a Swift Package with the code above as the executable target's main.swift
2. Run swift build -c release && .build/release/<target> → crashes immediately with SIGSEGV (exit 139), no output
3. Run swift build && .build/debug/<target> → runs to completion, prints "Done"

Expected behavior

Both debug and release builds should run to completion.

Actual behavior

Release build (-O) crashes on the first call to buildSections with SIGSEGV before producing any output.

Workaround

Change the array type from [AnyObject?] to [SettingsSection?] and replace compactMap { $0 as? SettingsSection } with compactMap { $0 }. This produces identical runtime behavior and does not crash.

Environment

• Swift 6.3.1 (swiftlang-6.3.1.1.2, clang-2100.0.123.102)
• Apple Silicon (arm64-apple-macosx26.0)
• Optimization: -O (release); -Onone (debug) is not affected

Attachments

• Code to reproduce: SwiftAnyObjectCast.zip
• Output SIL for swiftc -Onone -emit-sil: noopt.sil.zip
• Output SIL for swiftc -O -emit-sil: opt.sil.zip