`unknown free variable` for recursive `grind` terms

[chenson2018]

Prerequisites

• Check that your issue is not already filed:
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• 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
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Description

Consider the following example:

@[grind =]
def f : Nat → Nat
  | 0 => 0
  | n + 1 => f n

theorem foo (n : Nat) : f n = 0 := by
  match n with
  | 0 => grind
  | n + 1 => grind [foo n]

On 4.27.0-rc1 the proof fails (correctly according to Kim), but has an uninformative unknown free variable ... error. Especially because a user might expect this to work following #11268, it would be nice to give an informative error message, maybe indicating to try using have with the provided term.

On 4.28.0-nightly-2026-01-07 however, this example and another provided on Zulip both seem to pass. Given that Kim mentioned this should fail, I decided to still open this issue.

Context

Reported at #lean4>grind in recursive proofs

Steps to Reproduce

1. Run the above example

Expected behavior: fail with a more informative error

Actual behavior: fails with a unknown free variable error

Versions

Version: 4.27.0-rc1, 4.28.0-nightly-2026-01-07
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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