Unused variable linter false positives

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

universe u

example (f : Type u → Type (u + 1)) :
    (Σ (X : Type u) (Y : Type u), Unit → X → Y) → Type (u + 1) :=
  fun (⟨X, Y, _⟩ : Σ (X : Type u) (Y : Type u), Unit → X → Y) ↦ (f X → f Y)
--                                                         ^ unused variable linter triggers here

#eval show Id _ from do
  let vals := #[1, 2]
  let rec go xs acc  := do
  --         ^  ^   unused variable linter triggers here
    if h : xs.size = 0 then
      acc
    else
      go xs[1:] (acc.push xs[0])
  go vals #[]

The first one is interesting because the linter triggers on something that doesn't even declare a variable.

The second one seems to be specific to #eval; if you change it into a partial def then the error goes away.

Context

Reported at #new members > "Unused variable" that is clearly not unused

Expected behavior: No unused variable warnings

Actual behavior: Unused variable warnings

Versions

Lean 4.31.0-nightly-2026-04-09 on live.lean-lang.org

Impact

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Start by reproducing both minimal examples in the Lean nightly environment, checking the reported unused-variable locations. Compare the #eval case with the partial def version and confirm that the fix eliminates both false-positive warnings without introducing new ones.