unexpected "dependent match elimination failed"

[b-mehta]

Prerequisites

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Description

The equation compiler gives error "dependent match elimination failed" when I expect the match to succeed

Context

inductive MyOrder : Nat × Nat × Nat → Nat × Nat × Nat → Prop
  | twice {x y n u v m : Nat} : m + 2 ≤ n → MyOrder (x, y, n) (u, v, m)
  | within {x y u v m : Nat} : x ≤ u → y ≤ v → MyOrder (x, y, m) (u, v, m)
  | next_min {x y u v m : Nat} : min x y + 1 ≤ min u v → MyOrder (x, y, m + 1) (u, v, m)

theorem antisymmetric : ∀ {x y : Nat × Nat × Nat}, MyOrder x y → MyOrder y x → x = y
  | _, _, .twice _, .twice _ => _
  | _, _, .twice _, .within _ _ => _
  | _, _, .twice _, .next_min _ => _
  | _, _, .within _ _, .twice _ => _
  | _, _, .within _ _, .within _ _ => _
  | _, _, .next_min _, .twice _ => _

Steps to Reproduce

1. Code as above

Expected behavior: Code gives errors on the six right-hand-sides to fill in the six proofs.

Actual behavior: The line | _, _, .twice _, .within _ _ => _ gives confusing error

dependent match elimination failed, inaccessible pattern found
  .(n✝)
constructor expected

Versions

Lean 4.15.0-rc1
Target: x86_64-unknown-linux-gnu

Using live.lean-lang.org on Lean nightly.

Additional Information

Discussion here: https://leanprover.zulipchat.com/#narrow/channel/270676-lean4/topic/dependent.20match.20elimination.20failed

Impact

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[kmill]

A bit more context: we found the following two fixes:

theorem antisymmetric : ∀ {x y : Nat × Nat × Nat}, MyOrder x y → MyOrder y x → x = y
  | _, _, .twice _, .twice _ => _
  | _, p, .twice _, .within _ _ => _
  | _, _, .twice _, .next_min _ => _
  | _, _, .within _ _, .twice _ => _
  | _, _, .within _ _, .within _ _ => _
  | _, _, .next_min _, .twice _ => _

and

theorem antisymmetric : ∀ {x y : Nat × Nat × Nat}, MyOrder x y → MyOrder y x → x = y
  | _, _, .twice _, .twice _ => _
  | _, (_,_,_), .twice _, .within _ _ => _
  | _, _, .twice _, .next_min _ => _
  | _, _, .within _ _, .twice _ => _
  | _, _, .within _ _, .within _ _ => _
  | _, _, .next_min _, .twice _ => _

This was by trial-and-error. (I also tried to write the complete patterns with all the necessary inaccessible patterns manually, but I didn't succeed.)

[cppio]

Another example that gives the same error:

inductive Pred : Nat → Nat → Type
  | here : Pred n.succ n
  | there : Pred n m → Pred n.succ m.succ

inductive HasPred n
  | mk (p : Pred n m)

def fails : HasPred (.succ n) → Unit
  | .mk .here => ()
  | .mk (.there _) => ()

def works : HasPred (.succ n) → Unit
  | .mk .here => ()
  | .mk (m := _) (.there _) => ()