PANIC at Lean.Meta.whnfEasyCases

Stream: lean4

[Scott Morrison]

lean4#1842 reports a PANIC that I encountered trying to bump std4/mathlib4 to the latest nightly, so I could finish logic.equiv.defs which is waiting on other fixes to lean4.

variable (R : α → α → Prop)

inductive List.Pairwise : List α → Prop
  | nil : Pairwise []
  | cons : ∀ {a : α} {l : List α}, (∀ {a} (_ : a' ∈ l), R a a') → Pairwise l → Pairwise (a :: l)

theorem and_assoc : (a ∧ b) ∧ c ↔ a ∧ (b ∧ c) :=
  ⟨fun ⟨⟨ha, hb⟩, hc⟩ => ⟨ha, hb, hc⟩, fun ⟨ha, hb, hc⟩ => ⟨⟨ha, hb⟩, hc⟩⟩

theorem and_left_comm : a ∧ (b ∧ c) ↔ b ∧ (a ∧ c) := by
  rw [← and_assoc, ← and_assoc, @And.comm a b]
  exact Iff.rfl

theorem pairwise_append {l₁ l₂ : List α} :
    (l₁ ++ l₂).Pairwise R ↔ l₁.Pairwise R ∧ l₂.Pairwise R ∧ ∀ {a} (_ : a ∈ l₁), ∀ {b} (_ : b ∈ l₂), R a b := by
  induction l₁ <;> simp [*, and_left_comm]

(The proof given here should have unsolved goals, but no panic.)

[Kevin Buzzard]

(This code seems to work in mathlib4, which is on 4.0.0-nightly-2022-11-14; the issue reports that it doesn't work on 4.0.0-nightly-2022-11-16)

[Gabriel Ebner]

This is a fallout from the fix for 1815, which was merged yesterday.

[Leonardo de Moura]

Pushed a fix for this.