Adjust Circuit.__add__ behavior to match Circuit.__iadd__ (#3364)

Fixes #3353. This is a breaking change: it modifies the moment structure produced by code of the form `circuit + gate`. Other circuit-addition protocols (e.g. `circuit + moment(s)` or `circuit + circuit`) are unaffected by this change.

The new test fails with the old `circuit + gate` behavior.

Author: 95-martin-orion

cirq/circuits/circuit.py

@@ -262,18 +262,14 @@ def __iadd__(self, other):
         return self
 
     def __add__(self, other):
-        if not isinstance(other, type(self)):
-            if not isinstance(other, (ops.Operation, Iterable)):
-                return NotImplemented
-            # Auto wrap OP_TREE inputs into a circuit.
-            other = Circuit(other)
-
-        device = (self._device if other.device is devices.UNCONSTRAINED_DEVICE
-                  else other.device)
-        device_2 = (other.device if self._device is devices.UNCONSTRAINED_DEVICE
-                    else self._device)
-        if device != device_2:
-            raise ValueError("Can't add circuits with incompatible devices.")
+        if isinstance(other, type(self)):
+            if (devices.UNCONSTRAINED_DEVICE not in [
+                    self._device, other.device
+            ] and self._device != other.device):
+                raise ValueError(
+                    "Can't add circuits with incompatible devices.")
+        elif not isinstance(other, (ops.Operation, Iterable)):
+            return NotImplemented
 
         result = self.copy()
         return result.__iadd__(other)

cirq/circuits/circuit_test.py

@@ -268,6 +268,15 @@ def test_radd_op_tree():
         _ = [cirq.X(cirq.NamedQubit('a'))] + c
 
 
+def test_add_iadd_equivalence():
+    q0, q1 = cirq.LineQubit.range(2)
+    iadd_circuit = cirq.Circuit(cirq.X(q0))
+    iadd_circuit += cirq.H(q1)
+
+    add_circuit = cirq.Circuit(cirq.X(q0)) + cirq.H(q1)
+    assert iadd_circuit == add_circuit
+
+
 def test_bool():
     assert not cirq.Circuit()
     assert cirq.Circuit(cirq.X(cirq.NamedQubit('a')))