Failed to generate equality theorems

Prerequisites

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  • Checked that your issue isn't already filed.
  • Reduced the issue to a self-contained, reproducible test case.

Description

Pasted from Zulip:

inductive Ty where
| star: Ty
notation " ✶ " => Ty.star

abbrev Context : Type := List Ty

inductive Lookup : Context → Ty → Type where
| z : Lookup (t :: Γ) t

inductive Term : Context → Ty → Type where
| var : Lookup Γ a → Term Γ a
| lam : Term (✶ :: Γ) ✶ → Term Γ ✶
| ap : Term Γ ✶ → Term Γ ✶ → Term Γ ✶

abbrev plus : Term Γ a → Term Γ a
| .var i => .var i
| .lam n => .lam (plus n)
| .ap (.lam _) m => plus m -- This case takes precedence over the next one.
| .ap l m => (plus l).ap (plus m)

-- I wish to be able to prove the following "by definition":
example : plus (.ap l m) = (plus l).ap (plus m) := by
  unfold plus
--^^ failed to generate equality theorems for `match` expression `plus.match_1`

Expected behavior: The plus definition gets unfolded. IIRC since it's recursive, it should be unfolded only once.

Actual behavior: The last line generates the following error message:

failed to generate equality theorems for `match` expression `plus.match_1`
Γ : Context
a : Ty
motive : Term Γ a → Sort u_1
n : Term ( ✶ :: Γ) ✶
h_1 : (i : Lookup Γ a) → motive (Term.var i)
h_2 : (n : Term ( ✶ :: Γ) ✶ ) → motive (Term.lam n)
h_3 : (a : Term ( ✶ :: Γ) ✶ ) → (m : Term Γ ✶ ) → motive (Term.ap (Term.lam a) m)
h_4 : (l m : Term Γ ✶ ) → motive (Term.ap l m)
⊢ Eq.rec (fun x motive h_1 h_2 h_3 h_4 h => (_ : Term.lam n = x) ▸ h_2 n) (_ : ✶ = a) (Term.lam n) motive h_1 h_2 h_3 h_4
      (_ : HEq (Term.lam n) (Term.lam n)) =
    h_2 n

Reproduces how often: 100%

Versions

lean-toolchain: leanprover/lean4:nightly-2023-05-16
OS: macOS Ventura 13.4 22F66 arm64

Start by reproducing the self-contained plus example with the stated Lean nightly version and inspect the failed equality-theorem generation for plus.match_1. Done means unfold plus succeeds for the example and the recursive definition unfolds once as expected, without the reported error.