`bv_decide` fails with `unknown free variable '_fvar.126'` under certain contexts

[Mr-Pine]

Prerequisites

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Description

bv_decide fails (in some cases) if the context contains a hypothesis of the form ∀ k : α, P k → <something about BitVecs>.

Context

Bisection on nightly versions showed this as a regression in nightly-2026-08-04 (#14215 looks like the likely culprit).

Steps to Reproduce

import Std.Tactic.BVDecide

example (y : BitVec 256) : True := by
  let x : BitVec 256 := y
  have h : ∀ k : Nat, k < 32 → x.toNat = x.toNat := fun _ _ => rfl
  have t : (0#64) ≤ (1#64) := by bv_decide
  trivial

or Lean Live Link

Expected behavior: bv_decide should prove t

Actual behavior: bv_decide fails with

unknown free variable `_fvar.126`

Versions

Lean 4.36.0-nightly-2026-09-16
Target: x86_64-unknown-linux-gnu Linux

Impact

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