Smart constructors for regex ranges: canonical form at construction time (#9814)

Regex range expressions (`re.range`) and Boolean operations over them
were left in unsimplified form, defeating downstream optimisations
(bisimulation classical fast-path, derivative engine) and producing
semantically-empty terms not syntactically equal to `re.none`.

## Changes

### `seq_decl_plugin.h` / `seq_decl_plugin.cpp`

- **`seq_util::rex::mk_range(sort*, unsigned lo, unsigned hi)`** — new
smart constructor that normalises at call time:
  - `lo > hi` → `re.empty`
  - `lo == hi` → `str.to_re` (singleton string)
  - `lo < hi` → `re.range`
- **`mk_info_rec` `OP_RE_RANGE`** — concrete non-empty ranges (both
bounds are single-char literals with `lo ≤ hi`) now return `classical =
true`, enabling the XOR-bisimulation `classical_distinguishing`
fast-path on character-predicate leaves. Symbolic/unknown ranges retain
`classical = false`.

### `seq_rewriter.cpp`

- **`mk_re_range`** — singleton collapse: `(re.range "a" "a")` →
`(str.to_re "a")`
- **`mk_regex_inter_normalize`** — range × range intersection: `[a,b] ∩
[c,d]` → `[max(a,c), min(b,d)]`, or `re.none` (disjoint), or `str.to_re`
(boundary singleton); now delegates to `re().mk_range(sort*, lo, hi)`
- **`mk_regex_union_normalize`** — range × range union for
overlapping/adjacent ranges: `[a,b] ∪ [c,d]` → `[min(a,c), max(b,d)]`;
disjoint ranges fall through to existing `merge_regex_sets`; now
delegates to `re().mk_range(sort*, lo, hi)`
- **`mk_re_complement`** — range complement expands to one or two
concrete ranges instead of an opaque `re.comp` node; now delegates to
`re().mk_range(sort*, lo, hi)`:
  - `comp([0, b])` → `[b+1, max]`
  - `comp([a, max])` → `[0, a-1]`
  - `comp([a, b])` → `[0, a-1] ∪ [b+1, max]`

```
(simplify (re.range "z" "a"))                              ; → re.none
(simplify (re.range "a" "a"))                              ; → (str.to_re "a")
(simplify (re.inter (re.range "a" "z") (re.range "f" "k"))); → (re.range "f" "k")
(simplify (re.union (re.range "a" "f") (re.range "g" "k"))); → (re.range "a" "k")
(simplify (re.comp  (re.range "b" "y")))                   ; → (re.union [0,a] [z,max])
```

### Tests

New `src/test/seq_rewriter.cpp` with 14 cases covering all the above
reductions plus downstream propagation (star/concat/union/inter
absorbing empty ranges).

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/test/seq_rewriter.cpp

@@ -0,0 +1,193 @@
+/*++
+Copyright (c) 2024 Microsoft Corporation
+
+Regression tests for seq_rewriter smart constructors for regex ranges.
+
+Tests:
+  1. Empty range (lo > hi) → re.none
+  2. Singleton range (lo == hi) → str.to_re lo
+  3. Range ∩ Range → reduced range or re.none
+  4. Range ∪ Range → merged range for overlapping/adjacent
+  5. Complement of range → one or two ranges
+  6. Downstream operators absorb empty ranges correctly
+--*/
+
+#include "ast/ast_pp.h"
+#include "ast/reg_decl_plugins.h"
+#include "ast/rewriter/th_rewriter.h"
+#include "ast/seq_decl_plugin.h"
+#include <iostream>
+
+// Build a single-char string literal expression.
+static expr_ref mk_str(ast_manager& m, seq_util& su, unsigned c) {
+    return expr_ref(su.str.mk_string(zstring(c)), m);
+}
+
+void tst_seq_rewriter() {
+    ast_manager m;
+    reg_decl_plugins(m);
+    th_rewriter rw(m);
+    seq_util su(m);
+
+    sort* str_sort = su.str.mk_string_sort();
+    sort* re_sort  = su.re.mk_re(str_sort);
+
+    auto range = [&](unsigned lo, unsigned hi) -> expr_ref {
+        return expr_ref(su.re.mk_range(mk_str(m, su, lo), mk_str(m, su, hi)), m);
+    };
+
+    // Arbitrary regex variable for downstream tests.
+    app_ref R(m.mk_fresh_const("R", re_sort), m);
+
+    // -----------------------------------------------------------------------
+    // 1. Empty range (lo > hi) → re.none
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e = range('z', 'a');
+        rw(e);
+        std::cout << "empty range lo>hi: " << mk_pp(e, m) << "\n";
+        ENSURE(su.re.is_empty(e));
+    }
+
+    // -----------------------------------------------------------------------
+    // 2. Singleton range (lo == hi) → str.to_re lo
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e = range('a', 'a');
+        rw(e);
+        std::cout << "singleton range: " << mk_pp(e, m) << "\n";
+        expr* inner = nullptr;
+        ENSURE(su.re.is_to_re(e, inner));
+    }
+
+    // -----------------------------------------------------------------------
+    // 3. Range intersection: overlapping → smaller range
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_inter(range('a', 'z'), range('f', 'k')), m);
+        rw(e);
+        std::cout << "range inter overlapping: " << mk_pp(e, m) << "\n";
+        unsigned lo = 0, hi = 0;
+        ENSURE(su.re.is_range(e, lo, hi) && lo == 'f' && hi == 'k');
+    }
+
+    // -----------------------------------------------------------------------
+    // 4. Range intersection: disjoint → re.none
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_inter(range('a', 'f'), range('k', 'z')), m);
+        rw(e);
+        std::cout << "range inter disjoint: " << mk_pp(e, m) << "\n";
+        ENSURE(su.re.is_empty(e));
+    }
+
+    // -----------------------------------------------------------------------
+    // 5. Range intersection: touching at boundary → singleton (str.to_re "f")
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_inter(range('a', 'f'), range('f', 'z')), m);
+        rw(e);
+        std::cout << "range inter touching: " << mk_pp(e, m) << "\n";
+        expr* inner = nullptr;
+        ENSURE(su.re.is_to_re(e, inner));
+    }
+
+    // -----------------------------------------------------------------------
+    // 6. Range union: overlapping → merged range
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_union(range('a', 'f'), range('e', 'k')), m);
+        rw(e);
+        std::cout << "range union overlapping: " << mk_pp(e, m) << "\n";
+        unsigned lo = 0, hi = 0;
+        ENSURE(su.re.is_range(e, lo, hi) && lo == 'a' && hi == 'k');
+    }
+
+    // -----------------------------------------------------------------------
+    // 7. Range union: adjacent → merged range
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_union(range('a', 'f'), range('g', 'k')), m);
+        rw(e);
+        std::cout << "range union adjacent: " << mk_pp(e, m) << "\n";
+        unsigned lo = 0, hi = 0;
+        ENSURE(su.re.is_range(e, lo, hi) && lo == 'a' && hi == 'k');
+    }
+
+    // -----------------------------------------------------------------------
+    // 8. Range union: disjoint → stays as union
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_union(range('a', 'c'), range('m', 'z')), m);
+        rw(e);
+        std::cout << "range union disjoint (stays as union): " << mk_pp(e, m) << "\n";
+        ENSURE(!su.re.is_range(e));
+    }
+
+    // -----------------------------------------------------------------------
+    // 9. Range complement (general): no longer a complement node
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_complement(range('b', 'y')), m);
+        rw(e);
+        std::cout << "range comp general: " << mk_pp(e, m) << "\n";
+        ENSURE(!su.re.is_complement(e));
+    }
+
+    // -----------------------------------------------------------------------
+    // 10. Range complement (lo = 0): single range [hi+1, max]
+    // -----------------------------------------------------------------------
+    {
+        expr_ref lo_str(su.str.mk_string(zstring(0u)), m);
+        expr_ref hi_str(su.str.mk_string(zstring((unsigned)'f')), m);
+        expr_ref e(su.re.mk_complement(su.re.mk_range(lo_str, hi_str)), m);
+        rw(e);
+        std::cout << "range comp lo=min: " << mk_pp(e, m) << "\n";
+        ENSURE(!su.re.is_complement(e));
+        ENSURE(su.re.is_range(e));
+    }
+
+    // -----------------------------------------------------------------------
+    // 11. Downstream: (re.* (re.range "z" "a")) → str.to_re ""
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_star(range('z', 'a')), m);
+        rw(e);
+        std::cout << "star of empty range: " << mk_pp(e, m) << "\n";
+        expr* inner = nullptr;
+        // star of empty → epsilon (str.to_re "")
+        ENSURE(su.re.is_to_re(e, inner) && su.str.is_empty(inner));
+    }
+
+    // -----------------------------------------------------------------------
+    // 12. Downstream: concat absorbs empty range → re.none
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_concat(R, su.re.mk_concat(range('z', 'a'), R)), m);
+        rw(e);
+        std::cout << "concat absorbs empty range: " << mk_pp(e, m) << "\n";
+        ENSURE(su.re.is_empty(e));
+    }
+
+    // -----------------------------------------------------------------------
+    // 13. Downstream: union absorbs empty range → R
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_union(R, range('z', 'a')), m);
+        rw(e);
+        std::cout << "union absorbs empty range: " << mk_pp(e, m) << "\n";
+        ENSURE(e.get() == R.get());
+    }
+
+    // -----------------------------------------------------------------------
+    // 14. Downstream: inter absorbs empty range → re.none
+    // -----------------------------------------------------------------------
+    {
+        expr_ref e(su.re.mk_inter(R, range('z', 'a')), m);
+        rw(e);
+        std::cout << "inter absorbs empty range: " << mk_pp(e, m) << "\n";
+        ENSURE(su.re.is_empty(e));
+    }
+
+    std::cout << "tst_seq_rewriter: all tests passed\n";
+}