Invalidate QuantumCircuit.parameters cache on global-phase modifica…

…tion

Previously, replacing a parametric `global_phase` with a non-parametric
global phase would modify the `ParameterTable` without invalidating the
`parameters` cache that the circuit kept.

qiskit/circuit/quantumcircuit.py

@@ -2932,6 +2932,7 @@ def global_phase(self, angle: ParameterValueType):
         # called by some subclasses before the inner `_global_phase` is initialised.
         global_phase_reference = (ParameterTable.GLOBAL_PHASE, None)
         if isinstance(previous := getattr(self, "_global_phase", None), ParameterExpression):
+            self._parameters = None
             self._parameter_table.discard_references(previous, global_phase_reference)
 
         if isinstance(angle, ParameterExpression) and angle.parameters:

test/python/circuit/test_parameters.py

@@ -192,6 +192,24 @@ def test_get_parameter_global_phase(self):
         self.assertIs(qc.get_parameter("x"), x)
         self.assertIsNone(qc.get_parameter("y", None), None)
 
+    def test_setting_global_phase_invalidates_cache(self):
+        """Test that setting the global phase to a non-parametric value invalidates the `parameters`
+        cache of the circuit."""
+        x = Parameter("x")
+        qc = QuantumCircuit(0, global_phase=x)
+        self.assertEqual(qc.global_phase, x)
+        self.assertEqual(set(qc.parameters), {x})
+        qc.global_phase = 0
+        self.assertEqual(qc.global_phase, 0)
+        self.assertEqual(set(qc.parameters), set())
+
+        qc = QuantumCircuit(0, global_phase=0)
+        self.assertEqual(qc.global_phase, 0)
+        self.assertEqual(set(qc.parameters), set())
+        qc.global_phase = x
+        self.assertEqual(qc.global_phase, x)
+        self.assertEqual(set(qc.parameters), {x})
+
     def test_has_parameter(self):
         """Test the `has_parameter` method."""
         x = Parameter("x")