Joe Groff fa4dd15612 Sema: Correct composition of property wrappers. ...

Fixes rdar://problem/53407949 | SR-11138. Previously, we'd only look at the outermost property wrapper to decide whether the wrapped property's getter and setter were `mutating` (or exist at all). In reality, this requires considering the semantics of the composed accesses of each wrapper layer's
`wrappedValue` property. Fixing this systematically addresses a number of issues:

- As SR-11138 reported, composing a nonmutating-get-set wrapper ought to produce a composed wrapper
  that's nonmutating.

- We would previously allow a property wrapper with a mutating getter to be nested inside one with
  only a getter, even though the resulting implementation was unsound (because there's no mutable
  context for the inner wrapper to execute its get on.)

- Similarly, we would construct unsound setters in cases where the setter can't exist, such as when
  the nested wrapper isn't settable but the outer wrapper is.

2019-07-26 16:09:54 -07:00

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