Fix for issue #2777: it makes no sense to call simulator.run when ign…

…ore_measurement_results has been set to True (#3618)

Raises a ValueError for DensityMatrixSimulator.run when ignore_measurement_results=True.

Fixes #2777.

cirq/sim/density_matrix_simulator.py

@@ -143,10 +143,10 @@ def __init__(
                In the other case:
                >>> simulator = cirq.DensityMatrixSimulator(
                ...     ignore_measurement_results = True)
-               >>> result = simulator.run(circuit)
 
-               The measurement result will be the maximally mixed state
-               with equal probability for 0 and 1.
+               will raise a `ValueError` exception if you call `simulator.run`
+               when `ignore_measurement_results` has been set to True
+               (for more see https://github.com/quantumlib/Cirq/issues/2777).
         """
         if dtype not in {np.complex64, np.complex128}:
             raise ValueError(f'dtype must be complex64 or complex128, was {dtype}')
@@ -160,6 +160,9 @@ def _run(
         self, circuit: circuits.Circuit, param_resolver: study.ParamResolver, repetitions: int
     ) -> Dict[str, np.ndarray]:
         """See definition in `cirq.SimulatesSamples`."""
+        if self._ignore_measurement_results:
+            raise ValueError("run() is not supported when ignore_measurement_results = True")
+
         param_resolver = param_resolver or study.ParamResolver({})
         resolved_circuit = protocols.resolve_parameters(circuit, param_resolver)
         check_all_resolved(resolved_circuit)

cirq/sim/density_matrix_simulator_test.py

@@ -66,6 +66,16 @@ def test_invalid_dtype():
         cirq.DensityMatrixSimulator(dtype=np.int32)
 
 
+@pytest.mark.parametrize('dtype', [np.complex64, np.complex128])
+def test_run_with_ignore_measurement_results(dtype):
+    q0, q1 = cirq.LineQubit.range(2)
+    simulator = cirq.DensityMatrixSimulator(dtype=dtype, ignore_measurement_results=True)
+
+    circuit = cirq.Circuit(cirq.X(q0), cirq.X(q1), cirq.measure(q0))
+    with pytest.raises(ValueError, match="ignore_measurement_results = True"):
+        simulator.run(circuit)
+
+
 @pytest.mark.parametrize('dtype', [np.complex64, np.complex128])
 def test_run_no_measurements(dtype):
     q0, q1 = cirq.LineQubit.range(2)
@@ -109,7 +119,7 @@ def test_run_bit_flips(dtype):
 @pytest.mark.parametrize('dtype', [np.complex64, np.complex128])
 def test_run_bit_flips_with_dephasing(dtype):
     q0, q1 = cirq.LineQubit.range(2)
-    simulator = cirq.DensityMatrixSimulator(dtype=dtype, ignore_measurement_results=True)
+    simulator = cirq.DensityMatrixSimulator(dtype=dtype)
     for b0 in [0, 1]:
         for b1 in [0, 1]:
             circuit = cirq.Circuit(