Fix mpl conditional drawing (#3032)

* fix conditional drawing in mpl

* changelog

* add ref image

* lint

CHANGELOG.md

@@ -15,6 +15,9 @@ The format is based on [Keep a Changelog].
 
 ## [UNRELEASED]
 
+### Fixed
+-   Fixed a bug in drawing conditional gates with matplotlib circuit drawer.
+
 ### Removed
 
 -   Removed `DeviceSpecification` in favor of `PulseChannelSpec`. (\#3033)

qiskit/visualization/matplotlib.py

@@ -727,7 +727,7 @@ class MatplotlibDrawer:
                             ii in self._creg_dict]
                     mask = 0
                     for index, cbit in enumerate(self._creg):
-                        if cbit.reg == op.condition[0]:
+                        if cbit.register == op.condition[0]:
                             mask |= (1 << index)
                     val = op.condition[1]
                     # cbit list to consider

test/python/visualization/test_circuit_matplotlib_drawer.py

@@ -144,3 +144,29 @@ class TestMatplotlibDrawer(QiskitVisualizationTestCase):
             'visualization/references/matplotlib_long_name_ref.png')
 
         self.assertImagesAreEqual(ref_filename, long_name_filename)
+
+    @unittest.skipIf(not visualization.HAS_MATPLOTLIB,
+                     'matplotlib not available.')
+    @unittest.skip('Test is unreliable on some platforms')
+    @unittest.skipIf(os.name == 'nt', 'Rendered image differs on windows')
+    def test_conditional(self):
+        """Test that circuits with conditionals draw correctly
+        """
+        qr = QuantumRegister(2, 'q')
+        cr = ClassicalRegister(2, 'c')
+        circuit = QuantumCircuit(qr, cr)
+
+        # check gates are shifted over accordingly
+        circuit.h(qr)
+        circuit.measure(qr, cr)
+        circuit.h(qr[0]).c_if(cr, 2)
+
+        conditional_filename = self._get_resource_path('current_conditional_matplotlib_ref.png')
+        visualization.circuit_drawer(circuit, output='mpl',
+                                     filename=conditional_filename)
+        self.addCleanup(os.remove, conditional_filename)
+
+        ref_filename = self._get_resource_path(
+            'visualization/references/matplotlib_conditional_ref.png')
+
+        self.assertImagesAreEqual(ref_filename, conditional_filename)