[swift-dev] "Swift 4.1 or Swift 3.3"

[Chris Lattner]

> On Jan 5, 2018, at 4:19 PM, Jordan Rose via swift-dev <swift-dev at swift.org> wrote:
> 
> Hi, all. Swift 4.1 is off on its own branch and going well, but we never quite came up with an answer for a particular problem developers might have: "am I running a Swift 4.1 compiler?”.

I agree, this is getting bad.  Ted mentioned that something like __has_feature in clang is probably the best way to go, so people could check the specific thing they care about, instead of a set of global version numbers.

Another thing that could help is something along the lines of:

	#if swift_stdlib(>=5.0)

which would presumably be active in any language mode when the 5.0 standard library is available.  It would be even better to use a generalized availability system for this: the standard library version could be treated just like Foundation versions are handled, for example.

-Chris


> 
> #if swift(>=3.2)
> // Swift 3.2 (4.0 in compatibility mode)
> // Swift 3.3 (4.1 in compatibility mode)
> // Swift 4.0
> // Swift 4.1
> #endif
> 
> #if swift(>=3.3)
> // Swift 3.3 (4.1 compatibily mode)
> // Swift 4.0
> // Swift 4.1
> // this one is probably not very useful
> #endif
> 
> #if swift(>=4.0)
> // Swift 4.0
> // Swift 4.1
> #endif
> 
> #if ???
> // Swift 3.3
> // Swift 4.1
> #endif
> 
> I don't think this is going to come up a lot, but given that we do have changes to the standard library and to the language, I can see people wanting it. Right now the only way to do it is the rather unwieldy:
> 
> #if swift(>=4.1) || (swift(>=3.3) && !swift(>=4.0))
> print("new")
> #else
> print("old")
> #endif
> 
> Do we need something better here, or do you think people will be okay with this? I'm realizing I don't really know how many people try to keep their libraries working across Swift versions and run into compatibility issues. 
> 
> (Strictly speaking this problem is already present with Swift 4.0.2 with 3.2.2 compatibility mode, but that's much less likely to come up.)
> 
> Jordan
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