Fix: Skip UnitaryGate instances during BasisTranslator

crates/transpiler/src/passes/basis_translator/compose_transforms.rs

@@ -177,8 +177,6 @@ fn name_to_packed_operation(name: &str, num_qubits: u32) -> Option<PackedOperati
             _ => unreachable!(),
         };
         Some(inst.into())
-    } else if name == "unitary" {
-        unreachable!("Having a unitary result from an `EquivalenceLibrary is not possible")
     } else {
         None
     }

crates/transpiler/src/passes/basis_translator/mod.rs

@@ -223,7 +223,11 @@ fn extract_basis(circuit: &DAGCircuit, min_qubits: usize) -> AhashIndexSet<GateI
         basis: &mut AhashIndexSet<GateIdentifier>,
         min_qubits: usize,
     ) {
-        for (_, operation) in circuit.op_nodes(true) {
+        // We should skip any instances of UnitaryGate as these should be handled by UnitarySynthesis
+        for (_, operation) in circuit
+            .op_nodes(true)
+            .filter(|(_, inst)| !matches!(inst.op.view(), OperationRef::Unitary(_)))
+        {
             if circuit.get_qargs(operation.qubits).len() >= min_qubits {
                 basis.insert((operation.op.name().to_string(), operation.op.num_qubits()));
             }
@@ -252,7 +256,11 @@ fn extract_basis_target(
     qarg_mapping: Option<&HashMap<Qubit, Qubit>>,
 ) {
     let qarg_mapping = |q: &Qubit| qarg_mapping.map(|map| map[q]).unwrap_or(*q);
-    for (_, node) in dag.op_nodes(true) {
+    // We should skip any instances of UnitaryGate as these should be handled by UnitarySynthesis
+    for (_, node) in dag
+        .op_nodes(true)
+        .filter(|(_, inst)| !matches!(inst.op.view(), OperationRef::Unitary(_)))
+    {
         let qargs: &[Qubit] = dag.get_qargs(node.qubits);
         if qargs.len() < min_qubits {
             continue;

releasenotes/notes/fix-basis-panic-7cf361c5ed5912f6.yaml

@@ -0,0 +1,11 @@
+fixes:
+- |
+    Fixed error in :class:`.BasisTranslator` in which if a backend
+    defined a :class:`.UnitaryGate` as part of its :class:`.Target`'s
+    basis gate set, the transpiler pipeline would panic.
+
+    The pass now filters out :class:`.UnitaryGate`'s by instance
+    ensuring none will ever get processed as part of the basis set.
+
+    See `#15278 <https://github.com/Qiskit/qiskit/issues/15278>`__
+    and `#15733 <https://github.com/Qiskit/qiskit/issues/15733>`__.

test/python/transpiler/test_basis_translator.py

@@ -1314,3 +1314,27 @@ def test_basis_nested_control_flow_op_aer(self):
                 pass
 
         self.assertEqual(transpiled, expected)
+
+    @unittest.skipUnless(HAS_AER, "Aer backend required for simulation")
+    def test_basis_with_unitary_basis_aer(self):
+        """Test that a circuit with a unitary gate in its target basis gets
+        skipped by the BasisTranslator"""
+        from qiskit_aer import AerSimulator
+
+        qr = QuantumRegister(3, "q")
+        cr = ClassicalRegister(3, "c")
+        qc = QuantumCircuit(qr, cr)
+
+        qc.cx(0, 1)
+        qc.h(0)
+        qc.cx(0, 1)
+        qc.sx(0)
+        qc.dcx(0, 2)
+
+        backend = AerSimulator()
+        pm = generate_preset_pass_manager(
+            optimization_level=2, backend=backend, seed_transpiler=134
+        )
+        transpiled = pm.run(qc)
+
+        self.assertEqual(Operator(qc), Operator(transpiled))