Retroactive Conformances vs. Swift-in-the-OS

[Karl]

Apologies for the bump, but a recent thread reminded me of this issue: Foundation.Date conforming to Strideable iOS 13.

It has also come up a few more times since this thread:

@jrose

Retroactive conformances (i.e. conformances where your module owns neither the type nor the protocol) are dangerous no matter what if you depend on a library you can't pin to a specific version. That said, conformances to protocols you do own are perfectly reasonable.

I was wondering if it might be worth pitching some kind of warning/error for retroactive conformances involving resilient libraries.