dot ident pattern does not treat parameters as inaccessible

[cppio]

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 elaborating a constructor pattern, the parameters are treated as inaccessible.

example : List α → Bool
  | List.nil (α := α) => false
  | List.cons (α := α) .. => true

In this example, (α := α) is equivalent to (α := .(α)) and does not bind a new variable. However, switching to the dot ident notation does not have this behavior, causing the example to fail.

example : List α → Bool
  | .nil (α := α) => false
  | .cons (α := α) .. => true

Context

This difference in behavior is relied upon in a mathlib test:
https://github.com/leanprover-community/mathlib4/blob/a7fc949f1b05c2a01e01c027fd9f480496a1253e/MathlibTest/congr.lean#L176-L181
Changing the pattern .nil n to walk.nil n causes it to no longer succeed.

Steps to Reproduce

Run the code above with the dot ident notation.

Expected behavior: No errors, same as the code which uses the full constructor name.

Actual behavior: The following error:

type mismatch
  []
has type
  List α : Type ?u.19
but is expected to have type
  List α✝ : Type ?u.9

Versions

Lean 4.16.0-nightly-2025-01-07
This bug has been present since the introduction of the dot ident feature in e1fa9c1

Impact

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