duplicates:

• SR-5633 isKnownUniquelyReferenced(_:) should optionally consider weak/unowned references

Issue Description:

At the moment, isKnownUniquelyReferenced returns true if there's exactly one strong reference, no matter how many weak/unowneds there are.
In most cases this is not a huge issue because it's mostly used to check if a value type need's to CoW it's storage class. The class is normally not publicly accessible so there's no way random bits of code could create a weak/unowned reference. On the flip-side however that's a great argument to make it only returns true if there's one strong and no weak/unowned refs whatsoever as it won't change any correct code.

Long story short: We should change isKnownUniquelyReferenced to return true only iff there's one strong and no weak/unowned references.

EDIT: After reading SR-5633, it turns out that the existing behaviour might be useful so maybe we should have a new runtime call which has the semantics I describe here.