Temporarily mark time-reversing feedback as not supported

Mitigates #764

Author: Strilanc

src/stim/util_top/circuit_inverse_qec.cc

@@ -124,6 +124,15 @@ void CircuitFlowReverser::do_measuring_instruction(const CircuitInstruction &ins
     rev.undo_gate(inst);
 }
 
+void CircuitFlowReverser::do_feedback_capable_instruction(const CircuitInstruction &inst) {
+    for (GateTarget t : inst.targets) {
+        if (t.is_measurement_record_target()) {
+            throw std::invalid_argument("Time-reversing feedback isn't supported yet. Found feedback in: " + inst.str());
+        }
+    }
+    rev.undo_gate(inst);
+}
+
 void CircuitFlowReverser::do_simple_instruction(const CircuitInstruction &inst) {
     Gate g = GATE_DATA[inst.gate_type];
     rev.undo_gate(inst);
@@ -207,13 +216,8 @@ void CircuitFlowReverser::do_instruction(const CircuitInstruction &inst) {
         case GateType::ISWAP_DAG:
         case GateType::XCX:
         case GateType::XCY:
-        case GateType::XCZ:
         case GateType::YCX:
         case GateType::YCY:
-        case GateType::YCZ:
-        case GateType::CX:
-        case GateType::CY:
-        case GateType::CZ:
         case GateType::H:
         case GateType::H_XY:
         case GateType::H_YZ:
@@ -229,6 +233,13 @@ void CircuitFlowReverser::do_instruction(const CircuitInstruction &inst) {
         case GateType::HERALDED_PAULI_CHANNEL_1:
             do_simple_instruction(inst);
             return;
+        case GateType::XCZ:
+        case GateType::YCZ:
+        case GateType::CX:
+        case GateType::CY:
+        case GateType::CZ:
+            do_feedback_capable_instruction(inst);
+            break;
         case GateType::MRX:
         case GateType::MRY:
         case GateType::MR:

src/stim/util_top/circuit_inverse_qec_test.py

@@ -1,3 +1,4 @@
+import pytest
 import stim
 
 
@@ -151,3 +152,14 @@ def test_measurement_ordering_3():
     assert circuit.has_all_flows(flows, unsigned=True)
     new_circuit, new_flows = circuit.time_reversed_for_flows(flows)
     assert new_circuit.has_all_flows(new_flows, unsigned=True)
+
+
+def test_feedback():
+    c = stim.Circuit("""
+        R 1
+        M 1
+        CX rec[-1] 0
+    """)
+    with pytest.raises(ValueError):
+        c.time_reversed_for_flows([stim.Flow("Z0 -> Z0")])
+        # TODO: once feedback is supported verify the inv flow is correct