Fail fast on measurements in has_unitary (#5020)

* Fail fast on measurements in has_unitary

* Unit test now also uses circuit

* Implement has_unitary instead

* Remove unused imports

* Unit test that would blow up the memory

cirq-core/cirq/ops/measurement_gate.py

@@ -84,6 +84,9 @@ def key(self, key: Union[str, 'cirq.MeasurementKey']):
     def _qid_shape_(self) -> Tuple[int, ...]:
         return self._qid_shape
 
+    def _has_unitary_(self) -> bool:
+        return False
+
     def with_key(self, key: Union[str, 'cirq.MeasurementKey']) -> 'MeasurementGate':
         """Creates a measurement gate with a new key but otherwise identical."""
         if key == self.key:

cirq-core/cirq/ops/measurement_gate_test.py

@@ -59,6 +59,11 @@ def test_measure_init(num_qubits):
         cirq.MeasurementGate()
 
 
+def test_measurement_has_unitary_returns_false():
+    gate = cirq.MeasurementGate(1, 'a')
+    assert not cirq.has_unitary(gate)
+
+
 @pytest.mark.parametrize('num_qubits', [1, 2, 4])
 def test_has_stabilizer_effect(num_qubits):
     assert cirq.has_stabilizer_effect(cirq.MeasurementGate(num_qubits, 'a'))

cirq-core/cirq/ops/pauli_measurement_gate.py

@@ -83,6 +83,9 @@ def key(self, key: Union[str, 'cirq.MeasurementKey']) -> None:
     def _qid_shape_(self) -> Tuple[int, ...]:
         return (2,) * len(self._observable)
 
+    def _has_unitary_(self) -> bool:
+        return False
+
     def with_key(self, key: Union[str, 'cirq.MeasurementKey']) -> 'PauliMeasurementGate':
         """Creates a pauli measurement gate with a new key but otherwise identical."""
         if key == self.key:

cirq-core/cirq/ops/pauli_measurement_gate_test.py

@@ -47,6 +47,11 @@ def test_init(observable, key):
     assert cirq.qid_shape(g) == (2,) * len(observable)
 
 
+def test_measurement_has_unitary_returns_false():
+    gate = cirq.PauliMeasurementGate([cirq.X], 'a')
+    assert not cirq.has_unitary(gate)
+
+
 def test_measurement_eq():
     eq = cirq.testing.EqualsTester()
     eq.make_equality_group(

cirq-core/cirq/protocols/has_unitary_protocol_test.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import numpy as np
+import pytest
 
 import cirq
 
@@ -26,6 +27,29 @@ class No:
     assert not cirq.has_unitary(No())
 
 
+@pytest.mark.parametrize(
+    'measurement_gate',
+    (
+        cirq.MeasurementGate(1, 'a'),
+        cirq.PauliMeasurementGate([cirq.X], 'a'),
+    ),
+)
+def test_fail_fast_measure(measurement_gate):
+    assert not cirq.has_unitary(measurement_gate)
+
+    qubit = cirq.NamedQubit('q0')
+    circuit = cirq.Circuit()
+    circuit += measurement_gate(qubit)
+    circuit += cirq.H(qubit)
+    assert not cirq.has_unitary(circuit)
+
+
+def test_fail_fast_measure_large_memory():
+    num_qubits = 100
+    measurement_op = cirq.MeasurementGate(num_qubits, 'a').on(*cirq.LineQubit.range(num_qubits))
+    assert not cirq.has_unitary(measurement_op)
+
+
 def test_via_unitary():
     class No1:
         def _unitary_(self):