An unexpected `Sendable` pitfall

[Nickolas_Pohilets (Nickolas Pokhylets)]

IIUC, there are no plans to implement this.

Down the thread @Douglas_Gregor mentions that he was thinking of a limited solution that wouldn't actually work in general case:

@Douglas_Gregor

Oh, this is tricky. I was thinking of it as something we would do specifically for the casts themselves, because changing the identity of existential types (such as any Sendable & P is distinct from any P at runtime) can have far-ranging implications. But that means that my approach here won't fix this existing issue with marker protocols.