[Backport releases/v4.30.0] fix: make delta-derived Prop-valued instances theorems#13314
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[Backport releases/v4.30.0] fix: make delta-derived Prop-valued instances theorems#13314github-actions[bot] wants to merge 1 commit intoreleases/v4.30.0from
github-actions[bot] wants to merge 1 commit intoreleases/v4.30.0from
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This PR makes the delta-deriving handler create `theorem` declarations instead of `def` declarations when the instance type is a `Prop`. Previously, `deriving instance Nonempty for Foo` would always create a `def`, which is inconsistent with the behavior of a handwritten `instance` declaration. For example, given: ```lean def Foo (α : Type u) := List α deriving instance Nonempty for Foo ``` Before: `@[implicit_reducible] def instNonemptyFoo ...` After: `@[implicit_reducible] theorem instNonemptyFoo ...` The implementation checks `isProp result.type` after constructing the instance closure, and uses `mkThmOrUnsafeDef` for the Prop case (which also handles the unsafe fallback correctly). The noncomputable check is skipped for Prop-typed instances since theorems can freely reference noncomputable constants. Closes #13295 🤖 Prepared with Claude Code --------- Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com> (cherry picked from commit 30dca7b)
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Backport 30dca7b from #13304.