问题
I'm playing around with the Xcode 10 beta, and I noticed while doing compile-time checks of the Swift version number that projects with the Swift Language Version set to Swift 4 in their Build Settings are reporting as Swift 4.1.50 (betas 2, 3, and 4 do this; I didn't get a chance to test beta 1). This strikes me as... bizarre, to say the least. The current AppStore version of Xcode, Xcode 9.4.1, reports its version as Swift 4.1.2.
So... What happened to Swift 4.1.3 through 4.1.49? Or is this a bug that I should file? Or is the 50 significant in some way?
You can test this yourself with my test file, Swift Version Checker
回答1:
According to Jordan Rose on the Swift bug tracker:
This is the least bad answer we could come up with, given that we may still release more 4.1.x versions if there's, say, a bad security vulnerability in one of them. (This has happened before.) We needed a version number that was greater than any existing 4.1 version, but still less than 4.2.
We did consider other ideas, but this is the one we went with for maximum compatibility. We at least won't have this problem in the future thanks to SE-0212.
So, what I take from this is that there might still have to be an actual 4.1 release sometime in the future, but that would just be a security patch atop 4.1.2, so the logical number for that would be 4.1.3. Meanwhile, the pseudo-4.1 Swift that the Swift 4.2 compiler can digest needs its own number that also reflects it's a dialect of 4.1, and their conclusion was 4.1.50, which is safely far enough away from 4.1.2 to allow for as many security patches as they want.
来源：https://stackoverflow.com/questions/51429536/what-is-swift-4-1-50