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54f188160c
This PR solves three distinct issues with the handling of `ite`/`dite`,`decide`. 1) We prevent the simprocs from picking up `noncomputable`, `Classical` instances, such as `Classical.propDecidable`, when simplifying the proposition in `ite`/`dite`/`decide`. 2) We fix a type mismatch occurring when the condition/proposition is unchanged but the `Decidable` instance is simplified. 3) If we rewrite the proposition from `c` to `c'` and the evaluation of the original instance `Decidable c` gets stuck we try fallback path of of obtaining `Decidable c'` instance and evaluating it. This matters when the instance is evaluated via `cbv_eval` lemmas. --------- Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
39 lines
1.3 KiB
Lean4
39 lines
1.3 KiB
Lean4
set_option cbv.warning false
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/-!
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# A regression test against `cbv` picking up `Classical.propDecidable`,
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when re-synthesizing instances.
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When `cbv` encounters `decide P`, it simplifies the proposition `P`. If `P`
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unfolds (e.g. `IsEven 2` → `∃ k, 2 * k = 2`), `simpDecideCbv` tries to
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synthetize `Decidable` instance for the *unfolded* form. With `open Classical`,
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this was picking up `Classical.propDecidable` (which uses `choice`), replacing
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the original computable instance with one that cannot be evaluated.
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The code now contains a guard ensuring that the instance is not made of constants
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marked as `noncomputable`.
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-/
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-- A predicate wrapping an existential — has a computable `DecidablePred` instance,
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-- but the unfolded existential has none (except the classical one).
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def IsEven (n : Nat) : Prop := ∃ k, n = 2 * k
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instance : DecidablePred IsEven := fun n =>
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if h : n % 2 = 0 then
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.isTrue ⟨n / 2, by omega⟩
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else
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.isFalse (by intro ⟨k, hk⟩; omega)
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-- Works, using the provided `DecidablePred` instance.
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example : decide (IsEven 2) = true := by cbv
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example : decide (IsEven 3) = false := by cbv
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-- Should not fail, when `Classical.propDecidable` becomes available.
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open Classical in
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example : decide (IsEven 2) = true := by cbv
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open Classical in
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example : decide (IsEven 3) = false := by cbv
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