The actual design states: The package manager will use standard version numbering rules to determine precedence of language versions. For e.g. 5 > 4.2.1 > 4.2 > 4. ... When building a package, we will always select the Swift language version that is most close to (but not exceeding) a valid language version of the Swift compiler in use. If a package does not specify a Swift language version, the tools version of the manifest will be used to derive the value.