tree 8c92d2deaf86b8c2070b110d3f1a1c891ef4323c
parent b356afc93680b341b1b0e99441544128df127775
author Slava Pestov <spestov@apple.com> 1509776757 -0700
committer Slava Pestov <spestov@apple.com> 1509864716 -0700

Sema: Fix source compatibility break with default initialization of optional properties

In Swift 4, properties declared with a sugared Optional type,
like Int?, have a default value of nil. This can be observed
in two ways:

- Classes and structs get an implicit no-argument initializer
- Designated initializers don't have to initialize this property

Note that this did not apply in general to properties where
the type was spelled explicitly as Optional<Int>, etc, mostly
because of implementation restrictions -- when we check if a
type has implicit initializers, we have not realized types for
all stored property members yet, and doing so is not really
possible without the iterative decl checker.

However, in some cases, we *did* perform default initialization
for Optional<Int>, because of some code duplication and
divergent code paths.

A recent refactoring cleaned up some of the mess in this area,
but accidentally broke source compatibility with code that
relied on the broken Optional<Int> case.

Fix this by simulating the old behavior in -swift-version 4,
and preserving the more correct behavior in -swift-version 5.

Fixes <rdar://problem/35319847>.