`simp` uses `congr` lemmas that are not visible

Prerequisites

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

Private lemmas marked as @[congr] from imported modules are used by simp even though they are not visible, causing unknown constant errors.

Steps to Reproduce

1. Check out the Lake project in this branch: https://github.com/datokrat/lean-bug-reproductions/tree/bugs7
2. Open Reproductions/Basic.lean and observe the error.

Reproductions/Dependency.lean:

module

public def f : Nat → Nat := sorry

@[congr]
theorem t (h : m = n) : f m = f n := sorry

Reproductions/Basic.lean:

module

import Reproductions.Dependency

/-- error: Unknown constant `_private.Reproductions.Dependency.0.t` -/
#guard_msgs in
theorem u (h : m = n) : f m = f n := by
  simp [h]

Expected behavior:

simp should only use congr lemmas that are visible. (In the given example, theorem u should succeed because simp actually doesn't need to use a specialized congr lemma.)

Actual behavior:

An unknown constant error is caused by simp.

Versions

Lean 4.27.0-nightly-2025-11-30
Target: arm64-apple-darwin24.6.0 macOS

Impact

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Run the Lake project from the bugs7 branch and reproduce the failure in Reproductions/Basic.lean, using Reproductions/Dependency.lean to understand the imported private congr lemma. Trace simp's congr-lemma handling and ensure theorem u succeeds without an unknown constant for an invisible lemma; rerun the reproduction to verify the fix.