Fix segfault in LexiRoute lookahead on classical vertices (#2248)

Author: cleitonaugusto

TKET_VERSION

@@ -1 +1 @@
-2.1.101
+2.1.102

pytket/docs/changelog.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## Unreleased
+
+Fixes:
+
+- Fix segfault in `LexiRouteRoutingMethod` (and so `DefaultMappingPass`) when
+  a `ClExprOp` is reached while looking ahead to the next two-qubit slice.
+
 ## 2.18.4 (September 2026)
 
 Fixes:

pytket/tests/mapping_test.py

@@ -17,7 +17,7 @@
 
 from pytket import Circuit, OpType
 from pytket.architecture import Architecture
-from pytket.circuit import CircBox, Node, PhasePolyBox, Qubit, UnitID
+from pytket.circuit import Bit, CircBox, Node, PhasePolyBox, Qubit, UnitID
 from pytket.circuit.named_types import UnitIdMap
 from pytket.mapping import (
     AASLabellingMethod,
@@ -439,3 +439,20 @@ def test_BoxDecompositionRoutingMethod() -> None:
     )
     assert circ.valid_connectivity(arc, directed=False)
     assert len(circ.get_commands()) == 4
+
+
+def test_LexiRoute_with_clexpr_in_lookahead() -> None:
+    # https://github.com/Quantinuum/tket/issues/2180
+    circ = Circuit(3, 3)
+    circ.CX(0, 1)
+    circ.CX(2, 1)
+    circ.H(2, condition_bits=[2])
+    circ.add_clexpr_from_logicexp(Bit(1) | Bit(2), [Bit(0)])
+    circ.add_clexpr_from_logicexp(Bit(1) & Bit(0), [Bit(2)])
+    circ.CX(0, 2)
+    circ.CX(0, 2)
+    arc = Architecture([(0, 1), (1, 2)])
+    MappingManager(arc).route_circuit(
+        circ, [LexiLabellingMethod(), LexiRouteRoutingMethod()]
+    )
+    assert circ.valid_connectivity(arc, directed=False)

tket/src/Mapping/MappingFrontier.cpp

@@ -253,8 +253,10 @@ void MappingFrontier::advance_next_2qb_slice(unsigned max_advance) {
       }
       EdgeVec in_edges =
           this->circuit_.get_in_edges_of_type(vert, EdgeType::Quantum);
-      // More than 1 edge means we want to keep edges, so continue
-      if (in_edges.size() > 1) {
+      // More than 1 edge means we want to keep edges, so continue. No edge
+      // means a purely classical vertex (e.g. ClExprOp), which has no quantum
+      // boundary to advance.
+      if (in_edges.size() != 1) {
         continue;
       }
       // can guarantee that we update now as non-updating cases have been

tket/test/src/test_LexiRoute.cpp

@@ -21,6 +21,7 @@
 #include "tket/Mapping/LexiRouteRoutingMethod.hpp"
 #include "tket/Mapping/MappingManager.hpp"
 #include "tket/Mapping/Verification.hpp"
+#include "tket/Ops/ClExpr.hpp"
 #include "tket/Ops/ClassicalOps.hpp"
 #include "tket/Placement/Placement.hpp"
 #include "tket/Predicates/CompilationUnit.hpp"
@@ -2387,4 +2388,31 @@ SCENARIO("Lexi route produces incorrect bimaps") {
   REQUIRE(mm.route_circuit_with_maps(circ, config, maps));
   REQUIRE(check_permutation(circ, maps));
 }
+SCENARIO("Lexi route with ClExprOps in the lookahead slice") {
+  // segfault Github #2180: advance_next_2qb_slice read the quantum in-edge of
+  // a purely classical vertex, which has none.
+  Circuit circ(3, 3);
+  circ.add_op<unsigned>(OpType::CX, {0, 1});
+  circ.add_op<unsigned>(OpType::CX, {2, 1});
+  circ.add_conditional_gate<unsigned>(OpType::H, {}, {2}, {2}, 1);
+  ClExpr bit_or(ClOp::BitOr, {ClBitVar{0}, ClBitVar{1}});
+  circ.add_op<unsigned>(
+      std::make_shared<const ClExprOp>(
+          WiredClExpr(bit_or, {{0, 0}, {1, 1}}, {}, {2})),
+      {1, 2, 0});
+  ClExpr bit_and(ClOp::BitAnd, {ClBitVar{0}, ClBitVar{1}});
+  circ.add_op<unsigned>(
+      std::make_shared<const ClExprOp>(
+          WiredClExpr(bit_and, {{0, 0}, {1, 1}}, {}, {2})),
+      {1, 0, 2});
+  circ.add_op<unsigned>(OpType::CX, {0, 2});
+  circ.add_op<unsigned>(OpType::CX, {0, 2});
+  Architecture arc({{Node(0), Node(1)}, {Node(1), Node(2)}});
+  MappingManager mm(std::make_shared<Architecture>(arc));
+  std::vector<RoutingMethodPtr> config = {
+      std::make_shared<LexiLabellingMethod>(),
+      std::make_shared<LexiRouteRoutingMethod>()};
+  REQUIRE(mm.route_circuit(circ, config));
+  REQUIRE(respects_connectivity_constraints(circ, arc, false));
+}
 }  // namespace tket