fix: #26132 unsound Nothing cast in Mirror.fromProduct for F-bounded case class#26268
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halotukozak wants to merge 2 commits into
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fix: #26132 unsound Nothing cast in Mirror.fromProduct for F-bounded case class#26268halotukozak wants to merge 2 commits into
halotukozak wants to merge 2 commits into
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…sses For an F-bounded case class such as `case class FB[T <: B[T]](v: T)`, the synthesized `fromProduct` reconstructed the constructor type arguments by instantiating the type variables to their solved bounds. The constraint solver picks `Nothing` as the solution because the parameter occurs in its own upper bound, so the generated body cast product elements to `Nothing` and crashed at runtime with a ClassCastException. When the solved instance is `Nothing` and the parameter is genuinely F-bounded, fall back to the declared upper bound with self-references substituted by `Nothing` (e.g. `B[T]` becomes `B[Nothing]`), which erases to a sound cast for every legal instantiation. Fixes scala#26132. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Fixes #26132
How much have you relied on LLM-based tools in this contribution?
Extensively, for finding the root cause. Self-reviewed before the PR.
How was the solution tested?
New automated tests
i26132.scalaadded