Error when using public structure with private field of private type from a different module

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

When trying to use a public structure with a private field of a private type in a different module, the compiler complains about not being to see the type of the private field, even though it should be unneeded.

Context

I was attempting to compose structures with private fields (for information hiding) across modules.

Steps to Reproduce

In Test.Basic:

module

private def Alias := Nat

public structure Wrapper where
  private val : Alias

In Test.Other:

module

public import Test.Basic

/--
error: Unknown constant `_private.Test.Basic.0.Alias`
-/
#guard_msgs in
def wrapperId (wrapper : Wrapper) : Wrapper := wrapper

Expected behavior:
The definition of wrapperId should be accepted by the compiler as Wrapper's definition is public and it never tries to access the private field val.

Actual behavior:
The compiler complains about being unable to see the private Alias definition, even though it shouldn't be needed for this function:

error: Unknown constant `_private.Test.Basic.0.Alias`

Versions

4.29.0-nightly-2026-02-05
Ubuntu 24.04.3 LTS, GNU/Linux 6.14.0-37-generic, x86_64

Impact

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Reproduce the issue using the Test.Basic and Test.Other modules, especially the wrapperId definition and its #guard_msgs check. Start by tracing how the compiler handles a public structure with a private field type across modules; done means wrapperId is accepted without the Unknown constant error.