`qe-lite` could produce malformed formulas when expanding bounded
quantifiers under nested binders, leaving outer de Bruijn indices
unshifted after eliminating an inner quantifier (e.g., `(:var 1)`
escaping capture). This change fixes index normalization in that rewrite
path and adds a regression for the reported forall/exists arithmetic
case.
- **Rewrite correctness in bounded quantifier expansion**
- In `src/qe/lite/qe_lite_tactic.cpp`, after substituting bounded
variables in payload conjuncts, apply `inv_var_shifter(num_decls)` so
outer bound variables are reindexed relative to the removed binder.
- This preserves quantifier structure correctness when
`try_expand_bounded_quantifier` eliminates an inner quantifier.
- **Regression coverage for the reported pattern**
- In `src/test/smt_context.cpp`, add a focused quantified arithmetic
formula matching the bug shape:
- outer `forall (x, x4)`
- inner `exists (y)`
- mixed inequalities that trigger qe-lite bounded expansion
- Assert the formula is unsatisfiable, preventing reintroduction of
invalid index handling in this path.
```c++
inst = vs(p, subst_map.size(), subst_map.data());
shift(inst, num_decls, inst); // reindex outer de Bruijn vars after eliminating inner quantifier
```
---------
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
