From 7d67dbfcfc81143bf35084e78543b658268afe37 Mon Sep 17 00:00:00 2001
From: Copilot <198982749+Copilot@users.noreply.github.com>
Date: Sun, 12 Jul 2026 18:53:15 -0700
Subject: [PATCH] Add QE regression for unsound `check-sat-using qe` result on
 quantified reals (#10097)

`check-sat-using qe` was reported to return `unsat` on a satisfiable
quantified-real formula, while a subsequent `check-sat` on the same
assertion returned `sat`. This PR adds focused regression coverage for
that shape to prevent reintroduction.

- **Regression coverage for the reported formula**
  - Added `test_qe_regression_4175()` in `src/test/quant_solve.cpp`.
  - Parses and quantifier-eliminates:
    ```smt2
    (forall ((b Real)) (= (= r1 b) (= b 0)))
    ```

- **Behavioral oracle encoded in the test**
  - Verifies the QE result is satisfiable under `r1 = 0`.
  - Verifies the QE result is unsatisfiable under `r1 != 0`.
- This captures the intended semantics of the original formula and
guards against the unsound `unsat` outcome from the QE path.

- **Integration**
- Wires the new regression into `tst_quant_solve()` so it runs with
existing quantifier-solver test coverage.

Example snippet from the new test logic:

```cpp
solver.assert_expr(result);
solver.assert_expr(m.mk_eq(r1, zero));
VERIFY(l_true == solver.check());

solver.assert_expr(result);
solver.assert_expr(m.mk_not(m.mk_eq(r1, zero)));
VERIFY(l_false == solver.check());
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
---
 src/test/quant_solve.cpp | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/src/test/quant_solve.cpp b/src/test/quant_solve.cpp
index 28667917fc..59e35d927e 100644
--- a/src/test/quant_solve.cpp
+++ b/src/test/quant_solve.cpp
@@ -168,6 +168,36 @@ static void test_quant_solver(ast_manager& m, char const* str, bool validate = t
     test_quant_solver(m, vars.size(), vars.data(), fml, validate);
 }
 
+static void test_qe_regression_4175() {
+    ast_manager m;
+    reg_decl_plugins(m);
+    smt_params params;
+    arith_util a(m);
+
+    expr_ref fml = parse_fml(m, "(forall ((b Real)) (= (= r1 b) (= b 0)))");
+    VERIFY(fml);
+    expr_ref result(m);
+    qe::expr_quant_elim qe(m, params);
+    qe(m.mk_true(), fml, result);
+    VERIFY(result);
+
+    expr_ref r1(m.mk_const(symbol("r1"), a.mk_real()), m);
+    expr_ref zero(a.mk_numeral(rational(0), false), m);
+
+    {
+        smt::kernel solver(m, params);
+        solver.assert_expr(result);
+        solver.assert_expr(m.mk_eq(r1, zero));
+        VERIFY(l_true == solver.check());
+    }
+    {
+        smt::kernel solver(m, params);
+        solver.assert_expr(result);
+        solver.assert_expr(m.mk_not(m.mk_eq(r1, zero)));
+        VERIFY(l_false == solver.check());
+    }
+}
+
 
 static void test_quant_solve1() {
     ast_manager m;
@@ -253,6 +283,7 @@ void tst_quant_solve() {
     disable_debug("heap");
 
     test_quant_solve1();   
+    test_qe_regression_4175();
 
 #if 0
     memory::finalize();
@@ -262,5 +293,3 @@ void tst_quant_solve() {
     exit(0);
 #endif
 }
-
-