[SR-14943] Fix-it for changing unsafeBitcast to unsafeDowncast should be more narrow or removed

[typesanitizer]

Previous ID	SR-14943
Radar	rdar://problem/80820402
Original Reporter	@typesanitizer
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, DiagnosticsQoI, StarterBug
Assignee	Rajagopalan (JIRA)
Priority	Medium

md5: 3d91228cea2823e2a2396d782c8b9edb

Issue Description:

According to the documentation, unsafeBitcast expects that the target type is layout-compatible with the type of the value passed in.

OTOH, unsafeDowncast expects that the type of the value is the target type (there is a _debugPrecondition(x is T)).

Currently, we have a fix-it where we recommend replacing unsafeBitcast with unsafeDowncast. However, strengthening a precondition like that in a fix-it is generally not OK unless we have some reason to believe that x is T will be true.

Here is a code example that can be tested in a playground:

import Foundation
import UIKit
​
@propertyWrapper
struct UnsafeBitCast<Actual: NSObjectProtocol, Mocked: NSObjectProtocol> {
    init(wrappedValue: Actual?, to mocked: Mocked.Type) {
        self.wrappedValue = wrappedValue
    }
    var wrappedValue: Actual?
    var projectedValue: Mocked? {
        wrappedValue.map({ unsafeBitCast($0, to: Mocked.self) }) // fix-it suggests unsafeDowncast
    }
}
​
class MockUIWindow: NSObject {
    @UnsafeBitCast(to: UIScene.self) var mockWindowScene: MockUIScene? = nil
    init(mockWindowScene: MockUIScene? = .init()) {
        self.mockWindowScene = mockWindowScene
    }
    var windowScene: UIScene? {
        $mockWindowScene
    }
}
​
class MockUIScene: NSObject {
    @objc var activationState: UIScene.ActivationState
    init(activationState: UIScene.ActivationState = .foregroundActive) {
        self.activationState = activationState
    }
}
​
let mockWindow = MockUIWindow()
print(mockWindow.windowScene!.activationState == .foregroundActive)

If one applies the fix-it, this code crashes due to the precondition failing. We should either remove the fix-it altogether or only apply it narrowly in the cases where we know for sure that x is T will be true.

[typesanitizer]

@swift-ci create

[swift-ci]

Comment by Rajagopalan Gangadharan (JIRA)

Hello theindigamer (JIRA User) Can I work on this if it's still open?

[typesanitizer]

Sure, go for it.

[swift-ci]

Comment by Rajagopalan Gangadharan (JIRA)

Hello @typesanitizer sorry for the late PR but here it is - #40523 .

[swift-ci]

Comment by Rajagopalan Gangadharan (JIRA)

@typesanitizer I am facing a problem which I don’t know how to resolve - https://forums.swift.org/t/swift-ios-build-error-due-to-sdk-versions/54373/2

can you help me out please?