incremental elaboration of partial mutual definitions under the module system

Prerequisites

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• Check that your issue is not already filed:
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• 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
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Description

Consider the following code snippet:

module

abbrev CollectM := IO

partial def collectConst : CollectM Unit := do
  return ()

partial def collectExpr : CollectM Unit := do
  return ()

public partial def collectConstraints : IO (List Nat) := do
  return []

If we now edit this to

module

abbrev CollectM := IO

mutual

partial def collectConst : CollectM Unit := do
  return ()

partial def collectExpr : CollectM Unit := do
  return ()

end

public partial def collectConstraints : IO (List Nat) := do
  return []

By inserting first the mutual then the end Lean will complain that:

Unknown identifier `CollectM`

Note: A private declaration `CollectM` (from the current module) exists but would need to be public to access here.

In other words it now believes that collectConst and collectExpr are public. However if I restart the file it correctly detects that they are in fact not so this is likely a result of incremental elaboration in combination with the public partial def below.

Expected behavior: incremental elaboration does not show false visibility errors

Actual behavior: it does

Versions

Lean 4.24.0-nightly-2025-09-10
Target: x86_64-unknown-linux-gnu

Impact

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Start with the minimal Lean snippet in the issue and reproduce it against Lean 4.24.0-nightly-2025-09-10 or the Lean nightly environment. Investigate incremental elaboration of the module containing the partial mutual definitions and later public partial definition; done means inserting mutual and end no longer produces the false CollectM visibility error.