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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

A screenshot of Xcode 10 beta 4 using the above-linked file to detect Swift 4.1.50 A screenshot of Terminal reproducing the bug using the <code>swift</code> command

Ky -
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    Looks like a bug, compare https://stackoverflow.com/questions/51307937/swift-version-build-configuration for a similar observation. – Martin R Jul 19 '18 at 18:30
  • Filed [SR-8323](https://bugs.swift.org/browse/SR-8323) – Ky - Jul 19 '18 at 19:08
  • It's a pseudo version, just like Swift 3.2, 3.3 and 3.4. 4.1.50 is the language version of a Swift 4.2 compiler running in Swift 4 compatibility mode. See https://forums.swift.org/t/se-0212-compiler-version-directive/12267/7 – Hamish Jul 19 '18 at 20:34
  • Yes, I know that, but why `50`? Why not `3`? – Ky - Jul 20 '18 at 14:28
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    Jordan Rose has now provided the rationale [in the comments of the bug report you filed](https://bugs.swift.org/browse/SR-8323?focusedCommentId=38063&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-38063) :) Feel free to write a self-answer, or I can write an answer if you want. – Hamish Jul 20 '18 at 15:10

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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.

Ky -
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