Bad error message "unknown parser declaration/category/alias `...`" when using a non-public syntax declaration from a public one.

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

This error message appears when using a non-public parser from a public one. This error message is confusing because the parser is known, and is similar cases with terms produce error messages that address the need for a public modifier.

It could also be remedied if the documentation on syntax would address this error explicitly and explain this case.

Context

Steps to Reproduce

1. Open a new lean file.
2. Paste the following:

module

syntax foo := ident
public syntax bar := foo

Expected behavior: An error explaining that syntax bar is public but uses a private parser foo

Actual behavior: The following error appears: "unknown parser declaration/category/alias foo"

Versions

Lean 4.32.0-nightly-2026-06-15
Target: x86_64-unknown-linux-gnu

Using live.lean-lang.org.

Start with the minimal Lean file from the Steps to Reproduce and run it against the latest nightly release or live.lean-lang.org. Trace how the public syntax bar := foo declaration resolves the private foo parser. Done means the diagnostic explains that public syntax bar uses a non-public parser, instead of reporting foo as unknown.