Backport "Fix derivesFrom false negative in provablyDisjointClasses" to 3.7.4
#23990
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
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. [Cherry-picked 5975a06]
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... [Cherry-picked 43af511]
[Cherry-picked 546fbd1]
Base automatically changed from
release-3.7.4_backport-23813
to
release-3.7.4
September 22, 2025 10:16
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Backports #23834 to the 3.7.4.
PR submitted by the release tooling.
[skip ci]