Fix handling of recursive bounds in constraints #9012
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These can lead to deep subtype errors, this also fixes scala#8976 but see also the next commit which fixes the underlying soundness issue.
`approximation` used to replace recursive occurences of the type variable being instantiated by `TypeBounds.empty`, but this is not sound in general since the approximation might be outside the bounds of the type variable. We could try to do something fancy with ApproximatingTypeMap but it seems complicated (and expensive) and there's an easier solution: since last commit, only the upper bound is allowed to be recursive, so we can just instantiate the type variable to its lower bound to avoid the problem in a sound and cheap way.
odersky
approved these changes
May 20, 2020
| @@ -87,6 +87,10 @@ trait ConstraintHandling[AbstractContext] { | |||
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| protected def addOneBound(param: TypeParamRef, bound: Type, isUpper: Boolean)(using AbstractContext): Boolean = | |||
| if !constraint.contains(param) then true | |||
| else if !isUpper && param.occursIn(bound) | |||
| // We don't allow recursive lower bounds when defining a type, | |||
| // so we shouldn't allow them as constraints either. | |||
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I am not 100% sure about performance implications here. Let's test this.
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test performance please |
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Yes, that looks like a much better way to fix #8976 |
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performance test scheduled: 1 job(s) in queue, 0 running. |
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Performance test finished successfully: Visit http://dotty-bench.epfl.ch/9012/ to see the changes. Benchmarks is based on merging with master (43e4bfa) |
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I think it's within the usual variance but we can run again to check |
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test performance please |
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performance test scheduled: 1 job(s) in queue, 0 running. |
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Performance test finished successfully: Visit http://dotty-bench.epfl.ch/9012/ to see the changes. Benchmarks is based on merging with master (43e4bfa) |
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