Why does this compile in Swift 6 language mode? (Xcode 16.3/Swift 6.1). Update: apparently a bug in the Swift 6.1 compiler. Swift 6.2 correctly raises errors.

isolation.swift

import Foundation

extension Thread {
    /// Helper for debugging (because Thread.current is noasync)
    static var currentThread: Thread {
        current
    }
}

nonisolated class NonSendable {
    var value = 0

    func increment() {
        value += 1
    }
}

nonisolated class Model {
    var state: NonSendable = .init()

    // Upcoming feature: "NonisolatedNonsendingByDefault"
    //nonisolated(nonsending) (in Swift 6.2, post SE-0461)
    func doSomethingWithCallerIsolation(isolation: isolated (any Actor)? = #isolation) async {
        print(#function, Thread.currentThread)
        // Mutate non-Sendable state
        self.state = NonSendable()
    }

    //@concurrent (in Swift 6.2, post SE-0461)
    func doSomethingOnGlobalExecutor() async {
        print(#function, Thread.currentThread)
        // Mutate non-Sendable state
        self.state.increment()
    }
}

// @concurrent
func freeNonisolatedFunction(model: Model) async {
    print(#function, Thread.currentThread)
    model.state = NonSendable()
}

@main
struct Main {
    @MainActor // make @MainActor explicit, but `main()` is always implicitly @MainActor
    static func main() async {
        MainActor.assertIsolated()
        print(#function, Thread.currentThread)
        // A local variable. What isolation does this have? @MainActor?
        let model = Model()
        // Works. Makes sense because we're staying on the current isolation.
        await model.doSomethingWithCallerIsolation()
        // Works. Why??? Aren't we sending the non-Sendable model from MainActor to the global executor?
        await model.doSomethingOnGlobalExecutor()
        // Works. Why??? Aren't we sending the non-Sendable model from MainActor to the global executor?
        await freeNonisolatedFunction(model: model)
        // Use model here to make sure region-based isolation can't send it
        print(#function, model)
    }
}

Turns out this is a bug in Swift 6.1. Compiling with a current 6.2 toolchain fails with the 2 expected sendability errors on lines 54 and 56.