こっちのPRが何か関係してますか？？ここら辺のコメントから何かしてそうだなあと思いまして

// Universally available type doesn't have any availability conditions // so it could be serialized into "unique" slot to safe space. // Check whether there are any conditionally available substitutions. // If there are, it means that "unique" we just read is a universally // available substitution.

https://github.com/apple/swift/pull/42167 (edited)

[Serialization] Opaque types: serialize conditionally available und...

…lying types Resolves: rdar://90595158