Fix basic gates not being marked as invertible (#282)

- _TurnGate was checking for SelfInverseGate instead of ReversibleGate
- HGate and SwapGate weren't implementing SelfInverseGate

Author: Strilanc

cirq/ops/common_gates.py

@@ -89,7 +89,7 @@ def trace_distance_bound(self):
 
     def try_cast_to(self, desired_type):
         if (desired_type in [gate_features.ExtrapolatableGate,
-                             gate_features.SelfInverseGate] and
+                             gate_features.ReversibleGate] and
                 self.can_extrapolate_effect()):
             return self
         if desired_type is gate_features.KnownMatrixGate and self.has_matrix():
@@ -240,6 +240,7 @@ def __hash__(self):
 
 class HGate(gate_features.AsciiDiagrammableGate,
             gate_features.CompositeGate,
+            gate_features.SelfInverseGate,
             gate_features.KnownMatrixGate,
             gate_features.SingleQubitGate):
     """180 degree rotation around the X+Z axis of the Bloch sphere."""
@@ -297,6 +298,7 @@ def __repr__(self):
 
 class SwapGate(gate_features.AsciiDiagrammableGate,
                gate_features.CompositeGate,
+               gate_features.SelfInverseGate,
                gate_features.KnownMatrixGate,
                gate_features.TwoQubitGate,
                InterchangeableQubitsGate):

cirq/ops/common_gates_test.py

@@ -138,7 +138,7 @@ def test_runtime_types_of_rot_gates():
         p = gate_type(half_turns=Symbol('a'))
         assert p.try_cast_to(ops.KnownMatrixGate) is None
         assert p.try_cast_to(ops.ExtrapolatableGate) is None
-        assert p.try_cast_to(ops.SelfInverseGate) is None
+        assert p.try_cast_to(ops.ReversibleGate) is None
         assert p.try_cast_to(ops.BoundedEffectGate) is p
         with pytest.raises(ValueError):
             _ = p.matrix()
@@ -150,7 +150,7 @@ def test_runtime_types_of_rot_gates():
         c = gate_type(half_turns=0.5)
         assert c.try_cast_to(ops.KnownMatrixGate) is c
         assert c.try_cast_to(ops.ExtrapolatableGate) is c
-        assert c.try_cast_to(ops.SelfInverseGate) is c
+        assert c.try_cast_to(ops.ReversibleGate) is c
         assert c.try_cast_to(ops.BoundedEffectGate) is c
         assert c.matrix() is not None
         assert c.extrapolate_effect(2) is not None

cirq/ops/reversible_composite_gate.py

@@ -39,7 +39,7 @@ def _reverse_operation(operation: raw_types.Operation,
 
 
 def inverse_of_invertible_op_tree(root: op_tree.OP_TREE,
-                                  extensions: Extensions = Extensions()
+                                  extensions: Extensions = None
                                   ) -> op_tree.OP_TREE:
     """Generates OP_TREE inverses.
 
@@ -50,6 +50,8 @@ def inverse_of_invertible_op_tree(root: op_tree.OP_TREE,
     Returns:
         An OP_TREE that performs the inverse operation of the given OP_TREE.
     """
+    if extensions is None:
+        extensions = Extensions()
     return op_tree.transform_op_tree(
         root=root,
         op_transformation=lambda e: _reverse_operation(e, extensions),

cirq/ops/reversible_composite_gate_test.py

@@ -104,3 +104,25 @@ def default_decompose(self, qubits):
             pass
 
     assert isinstance(Included(), ops.ReversibleCompositeGate)
+
+
+def test_works_with_basic_gates():
+    a = ops.NamedQubit('a')
+    b = ops.NamedQubit('b')
+
+    basics = [ops.X(a),
+              ops.Y(a)**0.5,
+              ops.Z(a),
+              ops.CZ(a, b)**-0.25,
+              ops.CNOT(a, b),
+              ops.H(b),
+              ops.SWAP(a, b)]
+    assert list(ops.inverse_of_invertible_op_tree(basics)) == [
+        ops.SWAP(a, b),
+        ops.H(b),
+        ops.CNOT(a, b),
+        ops.CZ(a, b)**0.25,
+        ops.Z(a),
+        ops.Y(a)**-0.5,
+        ops.X(a),
+    ]