Equation compiler fails to prove that match is exhaustive

Prerequisites

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• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
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Description

In this code

def g (x : Nat) (hx : x = 0 ∨ x = 1) : Nat :=
  match x with
  | 0 => 0
  | 1 => 1

we get an error message missing cases: (Nat.succ (Nat.succ _)), _. On the other hand, in the code

def g (x : Nat) (hx : x = 0 ∨ x = 1) : Nat :=
  match x with
  | 0 => 0
  | 1 => 1
  | x + 20 => False.elim (by omega)

(which doesn't handle the case (Nat.succ (Nat.succ _)) either), there is no error message.

Steps to Reproduce

1. Copy the above code into live.lean-lang.org

Expected behavior: Both snippets should produce no error message.

Actual behavior: Error message in the first snippet.

Versions

4.12.0-nightly-2024-08-26 on live.lean-lang.org

Impact

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Start by reproducing both snippets in live.lean-lang.org using the reported Lean nightly version, then trace the equation compiler's exhaustiveness-checking entry point. Done means the first snippet no longer reports a missing case while preserving correct exhaustiveness diagnostics for genuinely incomplete matches.