Fix derivesFrom false negative in provablyDisjointClasses
#23834
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Before the addition of `SymDenotation#mayDeriveFrom`, tests/neg/i17132.min.scala was unsoundly accepted without a type error because `R[T]` is reduced to `Any` where `T <: P[Any]` by testing `provablyDisjointClasses` before `P` is added as a baseClass of `Q`. Now, a recursion overflows occurs because: - reducing `R[T] := T match case Q[t] => R[t]; case ...` requires - proving `T` disjoint from `Q[t]` where `T <: P[Any]`, which asks - whether existsCommonBaseTypeWithDisjointArguments, which requires - normalizing the type arg to the base class P for the class Q, i.e. R[t] - ... In short, despite the use of the pending set in provablyDisjoint, diverging is still possible when there is "cycle" in the type arguments in the base classes, (and where the pending set is thus reset for a separate normalization problem). One could attempt to add some more logic to detect these loops s.t. they are considered "stuck", as opposed to reporting an error. But it isn't clear that there are any concrete motivations for this.
For some reason, the derivesFrom issue was only observable in the minimization. The original test case diverges similarly to the previous description, but only when attempting normalizing during CodeGen...
sjrd
reviewed
Aug 29, 2025
sjrd
approved these changes
Aug 29, 2025
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