`declare_syntax_cat` in private section causes "unknown constant" on import

[datokrat]

Prerequisites

• 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

When using declare_syntax_cat in a private section of a module, importing the module fails with an "unknown constant" error.

Steps to Reproduce

1. Check out the Lake project in this branch: https://github.com/datokrat/lean-bug-reproductions/tree/bugs8
2. Open Reproductions/Basic.lean and observe that Lean complains fails to import Reproductions.Dependency.lean.

Reproductions/Basic.lean:

module

public import Reproductions.Dependency

Reproductions/Dependency.lean:

module

declare_syntax_cat ebnf

Expected behavior:

No errors, module is imported successfully.

Actual behavior:

Error: `Unknown constant `_private.Reproductions.Dependency.0.ebnf.quot`

There is a workaround: The error disappears if declare_syntax_cat is wrapped in a public section.

Versions

Lean 4.28.0-nightly-2025-12-28
Target: arm64-apple-darwin24.6.0 macOS

Impact

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