[swift-dev] Fixing complation condition processing

[Robert Widmann]

I think the source breakage here, if any in actual codebases, is minor enough that the patch could go through. 

~Robert Widmann

> On Mar 8, 2017, at 2:11 AM, rintaro ishizaki via swift-dev <swift-dev at swift.org> wrote:
> 
> Hi all,
> 
> I'm fixing #if condition processing in this PR
> https://github.com/apple/swift/pull/7955 <https://github.com/apple/swift/pull/7955>
> 
> It resolves these issues:
> 
> A) SR-3663 <https://bugs.swift.org/browse/SR-3663>: Precedence of '&&' and '||' and short-circuit rule of them.
> 
> #if false || true && false
>   print("true")
> #endif
> 
> This used to evaluate to 'true'.
> 
> B) SR-4032 <https://bugs.swift.org/browse/SR-4032>: Version check with binary operator.
> 
> #if swift(>=5.0) && FOO
>   Some Swift 5 code here
> #endif
> 
> I believe, this block should not be parsed.
> 
> C) SR-3663 <https://bugs.swift.org/browse/SR-3663>: Invalid condition after short-circuit binary operator.
> 
> #if true || true * This is not <acceptable>!
> #endif
> 
> This used to be accepted.
> 
> D) SR-4031 <https://bugs.swift.org/browse/SR-4031>: Compound name in the condition
> 
> #if FOO(_:)
> #elseif os(foo:)(macOS)
> #elseif os(Linux(foo:bar:))
> #endif
> 
> This used to be accepted.
> 
> In the PR, I keep A) and B) behavior in -swift-version 3 mode.
> However, as for C) and D), I don't think it's worth to keep them even in Swift3 mode, because they are obviously wrong.
> 
> What do you think? Do we need accept them in Swift3 mode?
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