Unknown constant error for privately imported unification hint

Prerequisites

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• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
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Description

If a unification hint is only in the private environment (because of a private import or because the hint itself is private) but the definitional equality check tries to use it in a public context, it fails with an error message "Unknown constant hint".

Context

#14532

Steps to Reproduce

module

public def MyType (_ : Nat) := Nat

local unif_hint where ⊢ MyType 0 =?= Nat

-- Error: Unknown constant `_private...hint✝`
public def test (x : MyType 0) (y : Fin (by with_reducible exact x)) : Nat := 3

Expected behavior:
The definitional equality check simply fails and we get an error message of the shape ... was expected to have type ....

Actual behavior:
We get an error message "Unknown constant _private...hint✝"

Versions

Lean 4.34.0-nightly-2026-08-09

Impact

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The issue names no files or tests. Start by running the self-contained Lean reproduction against the reported nightly version, then trace the definitional equality check and private unification-hint handling; done means the reproduction reports the expected type-mismatch error rather than an unknown private constant.