Fix #1812, Symbols.mapSymbols shouldn't replace denotations #1832
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It will use lazy types instead. The current version transforms a type, with a context that has denotations that may be forcefully replaced by mapSymbols. Types created during this transformation may cache denots, that are-to-be replaced. This is very problematic as this method is called from TreeTypeMap.withMappedSyms in a fixed-point cycle, creating new symbols on every iteration. Those cached denotations could make types keep symbols from previous iterations indefinitely. The changed version does not transform the types eagerly, and instead makes them lazy. Assuming there are no cycles, this should ensure correct ordering. Unfortunatelly, at this point in the compiler we basically always touch everything, and we can't even transform the info of denotation without this denotations info. We basically have a chicked&egg problem here. To solve it, I use the same trick as used by other lazy types by assigning an approximation of future type first. This allows to pass the tests and makes dotty more robust, but I suspect this isn't a complete fix and new similar bugs may arrive.
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Could you add the testcase from #1812 to verify that the fix work? |
…rmation. Reasoning similar to one in the previous commit also applies to annotations.
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@odersky please review. |
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LGTM 👍 |
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It will use lazy types instead. The current version transforms a type, with
a context that has denotations that may be forcefully replaced by
mapSymbols. Types created during this transformation may cache denots, that
are-to-be replaced. This is very problematic as this method is called from
TreeTypeMap.withMappedSyms in a fixed-point cycle, creating new symbols on
every iteration. Those cached denotations could make types keep symbols
from previous iterations indefinitely.
The changed version does not transform the types eagerly, and instead makes
them lazy. Assuming there are no cycles, this should ensure correct
ordering. Unfortunatelly, at this point in the compiler we basically always
touch everything, and we can't even transform the info of denotation
without this denotations info. We basically have a chicked&egg problem
here. To solve it, I use the same trick as used by other lazy types by
assigning an approximation of future type first. This allows to pass the
tests and makes dotty more robust, but I suspect this isn't a complete fix
and new similar bugs may arrive.