missing error for non-atomic pattern variable

Prerequisites

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• 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
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Description

Non-atomic variables (e.g. x.y) are normally disallowed in match patterns. If the non-atomic identifier exists in the environment, then the error no longer occurs.

Steps to Reproduce

def x.y := ()

example : Nat → Nat | x.y => x.y

Expected behavior: The error invalid pattern variable, must be atomic. This error appears if def x.y is removed.

Actual behavior: No error.

Versions

Lean 4.16.0-nightly-2025-01-05

Impact

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Start with the minimal reproducer in the issue: define x.y, then use x.y in the match pattern. Compare its behavior with the version where the definition is removed, and verify that the non-atomic pattern variable produces invalid pattern variable, must be atomic without changing valid patterns.