The source code compile as it is. But if you run swiftformat . (latest versions 0.49.4) at the root it change the other.swift file to something that don't compile anymore.
The rule redundantSelf is not correctly doing its work here. The self is needed to access via @dynamicMemberLookup the properties.
In the main file def.swift another private extension SomeType is correctly treated. I suppose that because it is in the same file as the definition using @dynamicMemberLookup it can be infered, but not when in another file ?
There is an option --selfrequired for function with @autoclosure arguments. But nothing similar for those dynamic member lookup.
This is either a bug, for not viewing that dynamic member lookup need the self, or it is a missing option to enumerate some properties where the rule redundantSelf shouldn't apply.