[SR-11749] Crash only on iOS when setting subscript for non-final class in other SwiftPM module

[swift-ci]

Previous ID	SR-11749
Radar	None
Original Reporter	mjm (JIRA User)
Type	Bug

Attachment: Download

Environment

❯ xcodebuild -version
Xcode 11.2
Build version 11B52

❯ swift --version
Apple Swift version 5.1.2 (swiftlang-1100.0.278 clang-1100.0.33.9)
Target: x86_64-apple-darwin19.0.0

Issue occurs on iOS 13.2 simulators and devices. Issue does not occur on macOS 10.15. Have not tested other situations.

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, Generics, Miscompile, RunTimeCrash
Assignee	None
Priority	Medium

md5: 41c97f70c67c490b9198ff1f08d9c0b4

duplicates:

• SR-11564 Swift 5.1.1 Regression: Bad access on generic types exported symbols – device only.

Issue Description:

I've hit an issue where a custom subscript I defined on a type in one of my Swift packages is causing a crash when running on iOS, but not on macOS (either with Catalyst or not).

I've attached a minimal reproduction package, which only shows the issue when running the tests in Xcode on an iOS simulator or device. In my real use case, this issue reproduces in both test cases and when using the package from an app.

I've reduced it down to a class shaped something like this:

public class Crashy<Friend> {
    public init() {}

    private var foo: String = "bar"

    var data: [String: Any] = [:]
    public subscript(key: String) -> Any? {
        get { data[key] }
        set {
            data[key] = newValue
        }
    }
}

The crash is triggered by creating a new instance and setting the subscript with any key/value.

The following details I've found to be important (removing any of them makes the crash go away):

• The class must be generic

• The class must have a private variable

• The class must be non-final

• The class must live in a module of a Swift package (the same use case with a class in an iOS framework linked by an app does not reproduce the issue)

I've seen the crash on a stack frame like this:

#0 0x00000001060a1099 in method descriptor for UserActions.Context.subscript.modify

In the attached test case, it's much weirder, as it's referencing a different type than the one the subscript is being called on:

#0 0x0000000102a3aba8 in direct field offset for NotCrashy.data ()

[beccadax]

I was able to reproduce this in Xcode 10.2.1, but not with a recent master compiler. This issue may have been fixed in Swift 5.2.

[beccadax]

@jckarter Look familiar?

[jckarter]

Doesn't ring any bells for me, sorry. There is a subscript-adjacent miscompile bug but the one I know about only affects the master branch.

[swift-ci]

Comment by Matt Moriarity (JIRA)

I just hit another case of something like this with the Entwine Swift package. In this case, there's no subscript involved, but the class is non-final, generic, and has private variables. I'm seeing iOS crash trying to access a field from a totally different class, while macOS works just fine.

[aschwaighofer]

This is a duplicate to SR-11564.