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duplicates:

• SR-12350 Projected PropertyWrappers don't trigger didSet when their wrappedValue is mutated

Issue Description:

As part of an attempt to work around SR-12350, I wrote code that amounts to
```
$myWrappedProperty.mutatingFunction {
// note: this function does not escape.
myWrappedProperty = someValue
}
```
My setter for the projection was the traditional { self = newValue }.

Initially, I expected myWrappedProperty to have a value of someValue after this code was run. In fact, however, it had a value of someValue while mutatingFunction is executing, then reverts to its original value. This is extremely confusing; especially because didSet is not run for the assignment of the projection.

To be fair, it seems to me that the compiler is actually following the letter of the law; post projection it is a different variable being mutated. But since the documentation also implies that a goal of memory exclusivity is to avoid situations where the programmer's intent is ambiguous, it seems reasonable to consider adding checks for this case.

Using _myProperty instead of $myProperty does produce a crash.

There are zero references to "wrapper" in the language reference page on memory exclusivity. I think that even if this is considered "working as intended" there should still be some documentation of how it behaves; ideally in the page for PropertyWrappers.

I've attached a sample file demonstrating the issue.