[swift-evolution] Idea: Support #if os(Darwin) as shorthand for os(iOS) || os(OSX) || os(watchOS) || os(tvOS)

[Brent Royal-Gordon]

> • Is the target an Apple platform (iOS, OS X, tvOS, watchOS) or not? (Answers the question "can I import Foundation"?)

Well, Corelibs Foundation will muddy those waters.

> • Is the target a "common UIKit platform" (compiles on both iOS and tvOS) or not? (Answers the question "can I import UIKit"?)

Maybe test for the presence of a particular module?

	#if supports(Foundation)
	#if supports(UIKit)

Or allow conditional importing of a module?

	#if import UIKit
		// Only compiled if UIKit imported; all UIKit APIs usable here
	#elseif import AppKit
		// Only compiled if AppKit imported; all AppKit APIs usable here
	#endif
	// No AppKit or UIKit APIs usable here

-- 
Brent Royal-Gordon
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