cc @allevato

AFAIK, the swift-X.Y.Z-RELEASE tags are automatically created by some part of Swift's CI/release process? They're not something I've intentionally created; looking at the Package.swift file (https://github.com/apple/swift-format/blob/swift-5.4.2-RELEASE/Package.swift#L142), we can see that it's trying to depend on main for the SwiftSyntax dependency, which would definitely be wrong—the tag was just created based on whatever was in the main branch at that time.

In general for swift-format, you should only rely on the numbered tags (like 0.50400.0) that represent releases that we've explicitly created and pinned to the correct version of swift-syntax. They're differentiated on https://github.com/apple/swift-format/releases by having more release information than just the tag name. (Unfortunately GitHub still shows both on that page, but the one labeled "Latest Release" is the one you want, which may not be the top one.)

I'm not familiar enough with swift-ci (which we don't have working yet for swift-format) to know if there's a way to have it automatically update the Package.swift dependencies to use the one corresponding to the same tag, or how other projects with inter-repository dependencies manage this.