Cache the result of CircuitOperation.has_unitary (#7483)

Otherwise every time `CircuitOperation.has_unitary` is called,
`has_unitary_protocol` [decomposes the circuit into
operations](https://github.com/quantumlib/Cirq/blob/609d93dbc51a6608a0d0c3f5d50d51325052e027/cirq-core/cirq/protocols/has_unitary_protocol.py#L98)
and calls `has_unitary` on each. This is inefficient, especially if
`repetitions > 1`, because `has_unitary_protocol` looks at the circuit
with repetitions.

To ensure this change is correct, I wrote the tests first, and then made
the change.

Author: codrut3

cirq-core/cirq/circuits/circuit_operation.py

@@ -29,7 +29,7 @@
 import sympy
 
 from cirq import circuits, ops, protocols, study, value
-from cirq._compat import proper_repr
+from cirq._compat import cached_method, proper_repr
 
 if TYPE_CHECKING:
     import cirq
@@ -296,12 +296,13 @@ def _qid_shape_(self) -> tuple[int, ...]:
     def _is_measurement_(self) -> bool:
         return self.circuit._is_measurement_()
 
+    @cached_method
     def _has_unitary_(self) -> bool:
         # Return false if parameterized for early exit of has_unitary protocol.
-        # Otherwise return NotImplemented instructing the protocol to try alternate strategies
         if self._is_parameterized_() or self.repeat_until:
             return False
-        return NotImplemented
+        operations = self._mapped_any_loop.all_operations()
+        return all(protocols.has_unitary(op) for op in operations)
 
     def _ensure_deterministic_loop_count(self):
         if self.repeat_until or isinstance(self.repetitions, sympy.Expr):

cirq-core/cirq/circuits/circuit_operation_test.py

@@ -22,7 +22,7 @@
 
 import cirq
 import cirq.circuits.circuit_operation as circuit_operation
-from cirq import _compat
+from cirq import _compat, protocols
 from cirq.circuits.circuit_operation import _full_join_string_lists
 
 ALL_SIMULATORS = (cirq.Simulator(), cirq.DensityMatrixSimulator(), cirq.CliffordSimulator())
@@ -1297,3 +1297,30 @@ def test_inner_repeat_until_simulate() -> None:
 
 
 # TODO: Operation has a "gate" property. What is this for a CircuitOperation?
+
+
+def test_has_unitary_protocol_returns_true_if_all_common_gates() -> None:
+    q = cirq.LineQubit(0)
+    op = cirq.CircuitOperation(cirq.FrozenCircuit(cirq.X(q), cirq.Y(q), cirq.Z(q)))
+    assert protocols.has_unitary(op)
+
+
+def test_has_unitary_protocol_returns_false_if_measurement_gate() -> None:
+    q = cirq.LineQubit(0)
+    key = cirq.MeasurementKey('m')
+    op = cirq.CircuitOperation(cirq.FrozenCircuit(cirq.X(q) ** 0.2, cirq.measure(q, key=key)))
+    assert not protocols.has_unitary(op)
+
+
+def test_has_unitary_protocol_returns_false_if_parametrized() -> None:
+    q = cirq.LineQubit(0)
+    exp = sympy.Symbol('exp')
+    op = cirq.CircuitOperation(cirq.FrozenCircuit(cirq.X(q) ** exp))
+    assert not protocols.has_unitary(op)
+
+
+def test_has_unitary_protocol_returns_true_if_all_params_resolve() -> None:
+    q = cirq.LineQubit(0)
+    exp = sympy.Symbol('exp')
+    op = cirq.CircuitOperation(cirq.FrozenCircuit(cirq.X(q) ** exp), param_resolver={exp: 0.5})
+    assert protocols.has_unitary(op)