i think the 'spirit' of this diagnostic is to deal with the fact that a Mutex can be populated with a non-Sendable value, but in order to do so it must be known to be in a 'disconnected' isolation region (i.e. is a sending value). since Mutex grants 'Sendability' to its contents, it must ensure that any subsequent changes to its contents do not alias non-Sendable values from other isolation regions.
as an simplified example, consider:
// non-Sendable type
class NS {}
final class C: Sendable {
let lock = Mutex(NS()) // okay to init since the initial value isn't in any isolation region
func setValue(_ v: NS) {
lock.withLock { existing in
// not okay to assign b/c `v` could have come from another isolation
// region so could introduce a data race
existing = v
} // 🛑
}
}
i feel like what you're 'supposed' to do in this circumstance is ensure the new value being assigned is also known to the compiler to be sending. AFAIK however, this for some reason still doesn't work currently (see a similar thread here).