unsound `unif_hint` can cause crash

Prerequisites

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• Reduce the issue to a minimal, self-contained, reproducible test case.
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Description

The following code causes Lean to crash:

unif_hint (x y : α) where ⊢ x ≟ y

inductive W {α : Type} (β : α → Type)
  | mk a (w : β a → W β)

I can't find documentation for unif_hint so I'm not sure what exactly it is supposed to do, but I think the unification hint should not be accepted.

Versions

Lean 4.22.0-nightly-2025-06-24

Impact

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Start with the minimal reproducer in the issue and test it against the specified Lean nightly version. Investigate how the unif_hint declaration is handled before the inductive W declaration; done means the example no longer crashes and the hint is rejected or handled safely.