[SR-14649] Incorrect warning - 'unsafeBitCast' from 'PartialKeyPath<Object>' to 'PartialKeyPath<T>' can be replaced with 'unsafeDowncast'

[swift-ci]

Previous ID	SR-14649
Radar	rdar://problem/78276246
Original Reporter	Olbo (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 1cb28da019f7958df1fc3fa446f6bad2

Issue Description:

The warning produced when trying to use unsafeBitCast and PartialKeyPath objects is incorrect, it is warning that it should be replaced with unsafeDowncast which is incorrect.

class Bar { var bar: Int = 42 }
class Baz: Bar { }

struct Foo<Object> {
    let baz: PartialKeyPath<Object>
    init(_ baz: PartialKeyPath<Object>) {
        self.baz = baz
    }
    func unsafeCast<T>(to _: T.Type) -> Foo<T> {
        .init(unsafeBitCast(baz, to: PartialKeyPath<T>.self)) // 'unsafeBitCast' from 'PartialKeyPath<Object>' to 'PartialKeyPath<T>' can be replaced with 'unsafeDowncast'
    }
    func peek(_ object: Object) -> Any {
        object[keyPath: baz]
    }
}


let foo = Foo(\Bar.bar)
let bar = Bar()
foo.peek(bar)
let baz = Baz()
foo.unsafeCast(to: Baz.self).peek(baz)

The reason I am trying this at all is that I wanted to mimic and use the super-type KeyPath on a subtype

baz[keyPath: \Bar.bar] // 42
baz[keyPath: \Baz.bar] // 42

[typesanitizer]

@swift-ci create

[xedin]

@jreference this one might not be too tricky as well.

[xedin]

Also could be dupe of #57285

[AnthonyLatsis]

Reduced examples:

class C1<T> {}

func test1<A, B>(_: A, x: C1<B>) {
  _ = unsafeBitCast(x, to: C1<A>.self)
}

class C2 {}

func test2<A: C2, B: C2>(_: A, x: B) {
  _ = unsafeBitCast(x, to: A.self)
}

[jreference]

https://github.com/apple/swift/blob/d3efca36dbda829b992931d04b814f98d7e3dae2/test/Constraints/suspicious_bit_casts.swift#L29-L32C2

Was looking at the tests related to this. I'm not sure why an arbitrary C<T> can be downcast to a subclass of C<Int>, it does not seem to be generally safe.

[xedin]

That’s why it’s unsafe :)