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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.
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