title: What versions of Swift are supported by what versions of Xcode?
body: <p>I develop apps in Swift for a living. I enjoy the language and follow it as closely as I can. Yet, still, certain facts slip through, whether by me being blindsided, or by Apple being very quiet about them.</p>
  
  <p>Today, I discovered that Swift 3.3 and 3.4 exist. I was under the assumption that 3.2 was the last version of 3, for use in migrating to 4, 4.2, and 5. This misconception is mostly due to the fact that <a href="https://swift.org/download/#snapshots" rel="noreferrer">Swift.org</a>, <a href="https://github.com/apple/swift/blob/master/CHANGELOG.md" rel="noreferrer">the official Git repo</a>, and <a href="https://developer.apple.com/library/archive/releasenotes/DeveloperTools/RN-Xcode/Chapters/Introduction.html" rel="noreferrer">Xcode Release Notes</a> don't mention them <em>at all</em>.</p>
  
  <p>So, I assume these are also transitional, but I've no idea what IDE or compiler they appear in, or what caveats they come with. Below is a table which sums up my knowledge:</p>
  
  <blockquote class="spoiler">
    <p> <h3>The below table is outdated, provided for context so the question makes sense. <a href="https://stackoverflow.com/a/52819598/3939277">I've posted an updated version of this table in an answer below.</a></h3>
  
   <a href="https://i.stack.imgur.com/2htFB.png" rel="noreferrer"><img src="https://i.stack.imgur.com/2htFB.png" alt="A complex table with many versions of Swift and Xcode as the axes, and the support versions as the content"></a></p>
  </blockquote>
  
  <p><strong>What goes in the purple spaces?</strong> Specifically for the 3.3 and 3.4 rows, but if you know the other ones, that would help too!</p>
  
  <p>Authoritative sources would also be awesome.</p>
  
  <hr>
  
  <p>Possible answers to the primary question are as follows:</p>
  
  <ol>
  <li>Swift 3.3 was only in Xcode 9.3 beta and never became public; Swift 3.4 was only in Xcode 9.3 - 9.4</li>
  <li>Swift 3.3 was only supported in Xcode 9.3; Swift 3.4 was supported in Xcode 9.3.1 and 9.4</li>
  <li>Swift 3.3 was supported in Xcode 9.3 and 9.3.1; Swift 3.4 was only supported in Xcode 9.4</li>
  <li>Swift 3.3 was supported in Xcode versions 9.3 though 9.4; Swift 3.4 was only supported in an Xcode 10 beta.</li>
  </ol>
  
  <p>I don't <em>think</em> there's another possibility. Do let me know if I'm off-base, though.</p>