Backport "Reduce projections of type aliases with class type prefixes" to LTS #21025
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Projections P # X are types that we would like to avoid. If X is a class type, there's nothing we can do. If X is an abstract type, we use skolemization and rewrite to (x?: P).X. If X is an alias type we should simply dealias but this was not done before. This caused an exonential blowup in #19892, where we costructed types of the form ZPartialServerEndpoint[R, A, B, I, E, O, -C] # EndpointType[A, I, E, T, R] ... # EndpointType[A, I, E, T, R] When the were 5 or more such selections, sompile times blew up (33s for 5, timeout after 5 minutes for 6). I am still not qute sure where the blowup happened. Looking at stacktraces of random interrups it seemed to be in a deep recursion of memberDenot and asSeenFrom calls.I believe it would still be interesting to find out more about this, in case there are other similar situations where combinations of deep projections with wide applications cannot be avoided. But for this precise problem, eagerly dealising fixes it. [Cherry-picked cd04d00][modified]
[Cherry-picked 291e84f]
[Cherry-picked 401f126][modified]
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No regressions detected in the community build up to lts-20295. |
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Backports #19931 to the LTS branch.
PR submitted by the release tooling.
[skip ci]