`reduce` does not recurse into binder types

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
  (You can also use the settings there to switch to “Lean nightly”)

Description

reduce does not recurse into binder types, even with skipTypes := false. As a result, the expression computed by reduce is not the input expression's normal form (at the given transparency).

MWE:

import Lean

open Lean Lean.Meta

def Foo := True

def Bar := Foo → Foo

run_meta do
  logInfo $ ← reduceAll (.const ``Bar [])
  -- Foo → True

Versions

"4.15.0-nightly-2024-11-14"

Impact

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Start with the minimal example in the issue and trace Lean.Meta.reduceAll, focusing on how binder types are handled when skipTypes := false. Confirm the current output and make the behavior recurse into those types so the result is the normal form, then rerun the reproducer.