matcher created in `def f := ...` body is private but occurs in type

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

Consider this situation:

module

public import Std
open Std

public section

def step : Nat × Nat → Nat × Nat
  | (x₁, x₂) => (x₂, x₁ + x₂)

def statesIterator := Iter.repeat (init := (0, 1)) step

/--
error: Unknown constant `_private.«external:file:///lean-nightly/lean-nightly.lean».0.step.match_1`

Note: A private declaration `step.match_1` (from the current module) exists but would need to be public to access here.
-/
#guard_msgs in
def fibIterator := statesIterator.map (fun (current, _) => current)

Here, the iterator fibIterator's type is inferred from its definition. This type also contains the lambda function in the map expression. Therefore, the matcher used by the lambda function, fibIterator.match_1, is going to be contained in fibIterator's type, which is public.

Expected behavior: [Clear and concise description of what you expect to happen]

No errors; fibIterator.match_1 should be public.

Actual behavior: [Clear and concise description of what actually happens]

Error: fibIterator.match_1 was made private and therefore can't occur in fibIterator's public type.

An even more minified example

module

/--
error: Unknown constant `_private.«external:file:///lean-nightly/lean-nightly.lean».0.vec.match_1`

Note: A private declaration `vec.match_1` (from the current module) exists but would need to be public to access here.
-/
#guard_msgs in
public def vec (n : Nat) := Vector.replicate (match n with | 0 => 0 | n' + 1 => n')

Versions

Lean 4.29.0-nightly-2026-02-14
Target: x86_64-unknown-linux-gnu

Impact

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Reproduce the issue with the minimal public def vec example and the step/fibIterator example using the Lean nightly version described, observing the #guard_msgs errors. Trace how the generated matcher is assigned visibility when it appears in an inferred public type; done means both examples elaborate without the private-declaration error.