Try/collapsed lubs #52
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Introduce new cases for AndTypes and OrTypes in methods `derivesFrom`, `baseClasses`, and `baseTypeWithArgs`. These cases are ultimately needed so that `baseTypeWithArgs` makes sense for union and intersection types. Also, fixed embarrassing typo in method `TypeBounds#|`.
Use the equality (where ~ is any form of refinement)
T1 { x ~ R1 } & T2 { x ~ R2 } == T1 & T2 { x ~ R1 & R2 }
We already did the same thing when distributing via "&".
Appromiximate union types by intersections of their common base classes. Controlled by option -Xkeep-unions. If option is set, no approximation is done. Motivations for approximating: There are two. First, union types are departure from Scala 2. From time to time they lead to failure of inference. One example experiences in Dotty was in a foldLeft, where the accumulator type was inferred to be Tree before and was now a union of two tree specific kinds. Tree was the correct type, whereas the union type was too specific. These failures are not common (in the Dotty codebase there were 3, I believe), but they cause considerable difficulty to diagnose. So it seems safer to have a compatibility mode with Scala 2. The second motivation is that union types can become large and unwieldy. A function like TreeCopier has a result type consisting of ~ 40 alternatives, where the alternative type would be just Tree. Once we gain more experience with union types, we might consider flipping the option, and making union types the default. But for now it is safer this way, I believe.
Removed annotations from the dotty codebase that were necessary because the inferred union types caused inference errors later on.
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Subsumed by #75 |
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See comment on third commit for motivation.