Address RuntimeError in qiskit_to_tk conversion.

.gitignore

@@ -11,4 +11,5 @@ dist
 obj
 docs/extensions
 .ipynb_checkpoints
+*.ipynb
 pytket/extensions/qiskit/_metadata.py

pytket/extensions/qiskit/qiskit_convert.py

@@ -500,7 +500,16 @@ def add_qiskit_data(self, data: "QuantumCircuitData") -> None:
                     gate_def = CustomGateDef.define(
                         instr.name, subc, list(subc.free_symbols())
                     )
-                    self.tkc.add_custom_gate(gate_def, params, qubits + bits)  # type: ignore
+                    try:
+                        self.tkc.add_custom_gate(gate_def, params, qubits + bits)  # type: ignore
+                    except RuntimeError:
+                        raise NotImplementedError(
+                            f"Conversion of qiskit's {instr.name} instruction is "
+                            + "currently unsupported by qiskit_to_tk. Consider "
+                            + "using QuantumCircuit.decompose() before attempting "
+                            + "conversion."
+                        )
+
             elif optype == OpType.CU3 and type(instr) == qiskit_gates.CUGate:
                 if instr.params[-1] == 0:
                     self.tkc.add_gate(

tests/qiskit_convert_test.py

@@ -26,7 +26,7 @@
 )
 from qiskit.quantum_info import Pauli, SparsePauliOp  # type: ignore
 from qiskit.transpiler import PassManager  # type: ignore
-from qiskit.circuit.library import RYGate, MCMT, XXPlusYYGate, PauliEvolutionGate, UnitaryGate  # type: ignore
+from qiskit.circuit.library import RYGate, MCMT, XXPlusYYGate, PauliEvolutionGate, UnitaryGate, RealAmplitudes  # type: ignore
 import qiskit.circuit.library.standard_gates as qiskit_gates  # type: ignore
 from qiskit.circuit import Parameter
 from qiskit.synthesis import SuzukiTrotter  # type: ignore
@@ -1013,3 +1013,18 @@ def test_failed_conversion_error() -> None:
         NotImplementedError, match=r"Conversion of qiskit's xx_plus_yy instruction"
     ):
         qiskit_to_tk(qc)
+
+
+def test_custom_gate_error() -> None:
+    qc = QuantumCircuit(3)
+
+    params = [np.pi / 2] * 9
+    real_amps1 = RealAmplitudes(3, reps=2)
+
+    real_amps2 = real_amps1.assign_parameters(params)
+    qc.compose(real_amps2, qubits=[0, 1, 2], inplace=True)
+
+    with pytest.raises(
+        NotImplementedError, match=r"Conversion of qiskit's RealAmplitudes instruction"
+    ):
+        qiskit_to_tk(qc)