The problem that it solves is that getTypeWithThisArgument call for the Bar1 creates a new intersection type, one that doesn't have any reference (through .target or similar) to the "original" intersection (the base type).

So createUnionOrIntersectionProperty creates 2 separate synthetic properties for a for both the base type and the derived type (even though they both refer to the same symbols, declarations, etc). That, in turn, fails derived === base check that is responsible for checking if the property was implemented or not.

This introduces some kind of a link between those types so I can trace this information back through the symbol links.

It's not a super elegant solution but it's the best I was able to figure out. I've tried to cache those property symbols and reuse them between both of those types but that led to a lot of problems. An alternative solution would be to make the mentioned derived === base check smarter but I also failed to figure out how to accurately compare 2 symbols like that.