Policy on hiding implementation details from Swift module interface

Hi all. I continue to work on bringing about this vision for a stable @_implementationOnly via abstract type layouts, but I've reached a point where I'd like to discuss the direction. Specifically, I want to discuss how much information we are willing to hide, potentially at the cost of diagnostics quality. Consider the following: Library Module: public class PublicClass { public init() {} internal func internalMethod() {} } Client Module: import Library func test(_ value: PublicClas...