[swift-evolution] #if os(Windows) and MSVC/Cygwin Compatibility

[Sangjin Han]

Hello swift-evolution,

This is continued from PR #2351(https://github.com/apple/swift/pull/2351).

Here is the brief history. (To avoid confusion, I used MSVC refer to
*-*-windows-msvc and Cygwin refer to *-*-windows-cygnus in LLVM.)

I needed the #if method to distinct MSVC from Cygwin, for mapping the Int
to CLongLong not CLong on MSVC.
In PR #2351, I simply added 'os(Cygwin)' and restrict 'os(Windows)' to
*-*-windows-msvc from *-*-windows-*, this solved my problem.

Jake(@jakepetroules) pointed out that Cygwin is not an OS and it will never
fixed to avoid breaking user applications.
There is more  participants and opinions, briefly,
- introduce another new one such as 'env(cygnus)' or 'triple(Cygwin)'
- the usability of the common condition 'os(Windows)' for *-*-windows-*
- fundamentally, what do we gain from asking which os() is in use?
- 'env()' is too vague
- what the right questions?

Forgive me the poor quotations of valuable opinions.


I hope we find out the solution or method everybody satisfied.


-Han Sangjin
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