Fix/issue 476 stack safety#795
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April 24, 2026 00:54
…-safe variants Default derivation (semiauto / *.derived / strict) again uses direct recursion, removing the ~5-9x performance regression introduced when the derivations were trampolined through Eval. Stack-safe (Eval-trampolined) derivation is now opt-in via cats.derived.stackSafe.* for Eq, Hash, Order, PartialOrder, Show, Functor, Contravariant, Invariant, Foldable, Reducible, Traverse and NonEmptyTraverse. Bifunctor, Bifoldable and Bitraverse are reverted to fast direct recursion (no Eval variant). Stack-safety tests updated to use the opt-in stackSafe entry points.
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Closes #476.
Fixes the StackOverflowError reported in #476 for derived Functor/Contravariant (and other typeclasses) on recursive ADTs. Default derivation (semiauto.* / Eq.derived / strict) keeps the existing fast direct-recursion path, so existing users see no behavior or performance change. A new opt-in entry point cats.derived.stackSafe.* provides Eval-trampolined variants for the 12 K0/K1 typeclasses; one import on the user side is enough to switch. Bifunctor/Bifoldable/Bitraverse keep fast direct recursion only (the two-given-set design doesn't fit the current K2-coproduct resolver in shapeless3 — happy to discuss a follow-up). All 12 081 existing tests pass; StackSafetySuite now covers all 12 K0/K1 instances via the opt-in entry point.