Avoid run_sweep_sample when subcircuits exist (#3745)

Author: daxfohl

cirq/sim/density_matrix_simulator.py

@@ -168,7 +168,9 @@ def _run(
         resolved_circuit = protocols.resolve_parameters(circuit, param_resolver)
         check_all_resolved(resolved_circuit)
 
-        if circuit.are_all_measurements_terminal():
+        if circuit.are_all_measurements_terminal() and not any(
+            circuit.findall_operations(lambda op: isinstance(op, circuits.CircuitOperation))
+        ):
             return self._run_sweep_sample(resolved_circuit, repetitions)
         return self._run_sweep_repeat(resolved_circuit, repetitions)
 

cirq/sim/density_matrix_simulator_test.py

@@ -1275,6 +1275,18 @@ def test_simulate_noise_with_terminal_measurements():
     assert result1 == result2
 
 
+def test_simulate_noise_with_subcircuit_measurements():
+    q = cirq.LineQubit(0)
+    circuit1 = cirq.Circuit(cirq.measure(q))
+    circuit2 = cirq.Circuit(cirq.CircuitOperation(cirq.Circuit(cirq.measure(q)).freeze()))
+
+    simulator = cirq.DensityMatrixSimulator(noise=cirq.X)
+    result1 = simulator.run(circuit1, repetitions=10)
+    result2 = simulator.run(circuit2, repetitions=10)
+
+    assert result1 == result2
+
+
 def test_density_matrix_copy():
     sim = cirq.DensityMatrixSimulator()