Ensure that cirq.decompose traverses the yielded OP-TREE in dfs ordering

cirq-core/cirq/ops/classically_controlled_operation.py

@@ -28,7 +28,7 @@
 import sympy
 
 from cirq import protocols, value
-from cirq.ops import raw_types
+from cirq.ops import op_tree, raw_types
 
 if TYPE_CHECKING:
     import cirq
@@ -105,11 +105,13 @@ def with_qubits(self, *new_qubits):
         )
 
     def _decompose_(self):
-        result = protocols.decompose_once(self._sub_operation, NotImplemented)
+        result = protocols.decompose_once(self._sub_operation, NotImplemented, flatten=False)
         if result is NotImplemented:
             return NotImplemented
 
-        return [ClassicallyControlledOperation(op, self._conditions) for op in result]
+        return op_tree.transform_op_tree(
+            result, lambda op: ClassicallyControlledOperation(op, self._conditions)
+        )
 
     def _value_equality_values_(self):
         return (frozenset(self._conditions), self._sub_operation)

cirq-core/cirq/ops/controlled_gate.py

@@ -28,7 +28,13 @@
 import numpy as np
 
 from cirq import protocols, value, _import
-from cirq.ops import raw_types, controlled_operation as cop, matrix_gates, control_values as cv
+from cirq.ops import (
+    raw_types,
+    controlled_operation as cop,
+    op_tree,
+    matrix_gates,
+    control_values as cv,
+)
 from cirq.type_workarounds import NotImplementedType
 
 if TYPE_CHECKING:
@@ -194,17 +200,17 @@ def _decompose_(
             return NotImplemented
 
         result = protocols.decompose_once_with_qubits(
-            self.sub_gate, qubits[self.num_controls() :], NotImplemented
+            self.sub_gate, qubits[self.num_controls() :], NotImplemented, flatten=False
         )
         if result is NotImplemented:
             return NotImplemented
 
-        decomposed: List['cirq.Operation'] = []
-        for op in result:
-            decomposed.append(
-                op.controlled_by(*qubits[: self.num_controls()], control_values=self.control_values)
-            )
-        return decomposed
+        return op_tree.transform_op_tree(
+            result,
+            lambda op: op.controlled_by(
+                *qubits[: self.num_controls()], control_values=self.control_values
+            ),
+        )
 
     def on(self, *qubits: 'cirq.Qid') -> cop.ControlledOperation:
         if len(qubits) == 0:

cirq-core/cirq/ops/controlled_operation.py

@@ -34,6 +34,7 @@
     eigen_gate,
     gate_operation,
     matrix_gates,
+    op_tree,
     raw_types,
     control_values as cv,
 )
@@ -145,7 +146,9 @@ def with_qubits(self, *new_qubits):
         )
 
     def _decompose_(self):
-        result = protocols.decompose_once_with_qubits(self.gate, self.qubits, NotImplemented)
+        result = protocols.decompose_once_with_qubits(
+            self.gate, self.qubits, NotImplemented, flatten=False
+        )
         if result is not NotImplemented:
             return result
 
@@ -154,13 +157,13 @@ def _decompose_(self):
             # local phase in the controlled variant and hence cannot be ignored.
             return NotImplemented
 
-        result = protocols.decompose_once(self.sub_operation, NotImplemented)
+        result = protocols.decompose_once(self.sub_operation, NotImplemented, flatten=False)
         if result is NotImplemented:
             return NotImplemented
 
-        return [
-            op.controlled_by(*self.controls, control_values=self.control_values) for op in result
-        ]
+        return op_tree.transform_op_tree(
+            result, lambda op: op.controlled_by(*self.controls, control_values=self.control_values)
+        )
 
     def _value_equality_values_(self):
         sorted_controls, expanded_cvals = tuple(

cirq-core/cirq/ops/gate_operation.py

@@ -160,7 +160,9 @@ def _num_qubits_(self):
         return len(self._qubits)
 
     def _decompose_(self) -> 'cirq.OP_TREE':
-        return protocols.decompose_once_with_qubits(self.gate, self.qubits, NotImplemented)
+        return protocols.decompose_once_with_qubits(
+            self.gate, self.qubits, NotImplemented, flatten=False
+        )
 
     def _pauli_expansion_(self) -> value.LinearDict[str]:
         getter = getattr(self.gate, '_pauli_expansion_', None)

cirq-core/cirq/ops/raw_types.py

@@ -830,7 +830,7 @@ def _json_dict_(self) -> Dict[str, Any]:
         return protocols.obj_to_dict_helper(self, ['sub_operation', 'tags'])
 
     def _decompose_(self) -> 'cirq.OP_TREE':
-        return protocols.decompose_once(self.sub_operation, default=None)
+        return protocols.decompose_once(self.sub_operation, default=None, flatten=False)
 
     def _pauli_expansion_(self) -> value.LinearDict[str]:
         return protocols.pauli_expansion(self.sub_operation)

cirq-core/cirq/protocols/decompose_protocol.py

@@ -160,7 +160,7 @@ def _decompose_dfs(item: Any, args: _DecomposeArgs) -> Iterator['cirq.Operation'
     decomposed = _try_op_decomposer(item, args.intercepting_decomposer)
 
     if decomposed is NotImplemented or decomposed is None:
-        decomposed = decompose_once(item, default=None)
+        decomposed = decompose_once(item, default=None, flatten=False)
 
     if decomposed is NotImplemented or decomposed is None:
         decomposed = _try_op_decomposer(item, args.fallback_decomposer)
@@ -275,12 +275,14 @@ def decompose_once(val: Any, **kwargs) -> List['cirq.Operation']:
 
 @overload
 def decompose_once(
-    val: Any, default: TDefault, *args, **kwargs
+    val: Any, default: TDefault, *args, flatten: bool = True, **kwargs
 ) -> Union[TDefault, List['cirq.Operation']]:
     pass
 
 
-def decompose_once(val: Any, default=RaiseTypeErrorIfNotProvided, *args, **kwargs):
+def decompose_once(
+    val: Any, default=RaiseTypeErrorIfNotProvided, *args, flatten: bool = True, **kwargs
+):
     """Decomposes a value into operations, if possible.
 
     This method decomposes the value exactly once, instead of decomposing it
@@ -296,6 +298,7 @@ def decompose_once(val: Any, default=RaiseTypeErrorIfNotProvided, *args, **kwarg
         *args: Positional arguments to forward into the `_decompose_` method of
             `val`.  For example, this is used to tell gates what qubits they are
             being applied to.
+        flatten: If True, the returned OP-TREE will be flattened to a list of operations.
         **kwargs: Keyword arguments to forward into the `_decompose_` method of
             `val`.
 
@@ -311,9 +314,8 @@ def decompose_once(val: Any, default=RaiseTypeErrorIfNotProvided, *args, **kwarg
     """
     method = getattr(val, '_decompose_', None)
     decomposed = NotImplemented if method is None else method(*args, **kwargs)
-
     if decomposed is not NotImplemented and decomposed is not None:
-        return list(ops.flatten_op_tree(decomposed))
+        return list(ops.flatten_to_ops(decomposed)) if flatten else decomposed
 
     if default is not RaiseTypeErrorIfNotProvided:
         return default
@@ -332,13 +334,16 @@ def decompose_once_with_qubits(val: Any, qubits: Iterable['cirq.Qid']) -> List['
 
 @overload
 def decompose_once_with_qubits(
-    val: Any, qubits: Iterable['cirq.Qid'], default: Optional[TDefault]
+    val: Any, qubits: Iterable['cirq.Qid'], default: Optional[TDefault], flatten: bool = True
 ) -> Union[TDefault, List['cirq.Operation']]:
     pass
 
 
 def decompose_once_with_qubits(
-    val: Any, qubits: Iterable['cirq.Qid'], default=RaiseTypeErrorIfNotProvided
+    val: Any,
+    qubits: Iterable['cirq.Qid'],
+    default=RaiseTypeErrorIfNotProvided,
+    flatten: bool = True,
 ):
     """Decomposes a value into operations on the given qubits.
 
@@ -355,6 +360,7 @@ def decompose_once_with_qubits(
             `_decompose_` method or that method returns `NotImplemented` or
             `None`. If not specified, non-decomposable values cause a
             `TypeError`.
+        flatten: If True, the returned OP-TREE will be flattened to a list of operations.
 
     Returns:
         The result of `val._decompose_(qubits)`, if `val` has a
@@ -366,7 +372,7 @@ def decompose_once_with_qubits(
         `val` didn't have a `_decompose_` method (or that method returned
         `NotImplemented` or `None`) and `default` wasn't set.
     """
-    return decompose_once(val, default, tuple(qubits))
+    return decompose_once(val, default, tuple(qubits), flatten=flatten)
 
 
 # pylint: enable=function-redefined

cirq-core/cirq/protocols/decompose_protocol_test.py

@@ -11,6 +11,8 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from typing import Optional
+from unittest import mock
 import pytest
 
 import cirq
@@ -308,3 +310,40 @@ def test_decompose_tagged_operation():
         'tag',
     )
     assert cirq.decompose_once(op) == cirq.decompose_once(op.untagged)
+
+
+def test_decompose_recursive_dfs():
+    class RecursiveDecompose(cirq.Gate):
+        def __init__(self, recurse: bool = True, mock_qm: Optional[mock.Mock] = None):
+            self.recurse = recurse
+            self.mock_qm = mock.Mock() if mock_qm is None else mock_qm
+
+        def _num_qubits_(self) -> int:
+            return 2
+
+        def _decompose_(self, qubits):
+            self.mock_qm.qalloc(self.recurse)
+            yield RecursiveDecompose(recurse=False, mock_qm=self.mock_qm).on(
+                *qubits
+            ) if self.recurse else cirq.Z.on_each(*qubits)
+            self.mock_qm.qfree(self.recurse)
+
+        def _has_unitary_(self):
+            return True
+
+    expected_calls = [
+        mock.call.qalloc(True),
+        mock.call.qalloc(False),
+        mock.call.qfree(False),
+        mock.call.qfree(True),
+    ]
+    mock_qm = mock.Mock(spec=["qalloc", "qfree"])
+    q = cirq.LineQubit.range(3)
+    gate = RecursiveDecompose(mock_qm=mock_qm)
+    gate_op = gate.on(*q[:2])
+    controlled_op = gate_op.controlled_by(q[2])
+    classically_controlled_op = gate_op.with_classical_controls('key')
+    for op in [gate_op, controlled_op, classically_controlled_op]:
+        mock_qm.reset_mock()
+        _ = cirq.decompose(op)
+        assert mock_qm.method_calls == expected_calls