PR見ると戻り値の型の部分だけ分かれているように見えますが、どうなんでしょう？？

If particular opaque type descriptor has multiple conditionally available underlying types, emit a special type and witness reference accessors that would be called at runtime to determine actual underlying type.

https://github.com/apple/swift/pull/42104

[IRGen] Augment opaque type descriptor to support limited availabil...

If particular opaque type descriptor has multiple conditionally available underlying types, emit a special type and witness reference accessors that would be called at runtime to determine actual u...