[swift-dev] Fixing complation condition processing

[rintaro ishizaki]

Hi all,

I'm fixing #if condition processing in this PR
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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