Unsoundness causing crash when capturing nonisolated local in escaping `@MainActor` closure

[KeithBauerANZ]

Description

No response

Reproduction

func escape(
    _ closure: @escaping @MainActor (String) -> Void
) {
    Task.detached { @MainActor in
        var i = 0
        while true {
            closure("MainActor \(i)")
            i += 1
        }
    }
}

func doTheThing() async {
    var x = ""
    escape {
        x = $0
    }
    var i = 0
    while true {
        x = "Nonisolated \(i)"
        i += 1
        print(x)
    }
}

await doTheThing()

swiftc -swift-version 6 NonisolatedLocalCapturedByMainActorClosure.swift && ./NonisolatedLocalCapturedByMainActorClosure
MainActor 210
MainActor 479
MainActor 502
(... tons more output)
MainActor 100354
Nonisolated 5010
Nonisolated 5011
Nonisolated 5012
Nonisolated 5013
Nonisolated 5014
Nonisolated 5015
Nonisolated 5016
Nonisolated 5017
Nonisolated 5018
Nonisolated 5019
Nonisolated 5020
Nonisolated 5021
Nonisolated 5022
Nonisolated 5023
Nonisolated 5024
zsh: segmentation fault  ./NonisolatedLocalCapturedByMainActorClosure

Expected behavior

There should be an error capturing the mutable nonisolated local in the @MainActor closure

Environment

swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
Target: arm64-apple-macosx15.0

Additional information

No response