unknown free variable in unreachable pattern

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
  (You can also use the settings there to switch to “Lean nightly”)

Description

The following:

def ohno (x : List (Fin 2)) : Nat :=
  match x with
  | [⟨0, _⟩] => sorry
  | [1] => sorry
  | [x] => sorry
  | _ => sorry

fails with unknown free variable '_fvar.113'.

Context

Discovered while helping with #new members > ✔ pattern matching on `Finset (Fin _)` @ 💬

Steps to Reproduce

1. Run the above code

Expected behavior: No error, or perhaps a warning that [x] is unreachable.

Actual behavior: The error reported above.

Versions

Lean 4.21.0-nightly-2025-06-02
Target: x86_64-unknown-linux-gnu

Additional Information

[Additional information, configuration or data that might be necessary to reproduce the issue]

Impact

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Start by running the minimal Lean example from the issue against the specified nightly version and confirm the unknown free variable error. Trace the pattern-matching and unreachable-pattern handling involved in the example; done means the example no longer reports an unknown free variable, with an unreachable-pattern warning being acceptable.