fix-decompose-hls-5019793177136024.yaml

---
features_circuits:
  - |
    Added a new argument ``"apply_synthesis"`` to :class:`.Decompose`, which allows
    the transpiler pass to apply high-level synthesis to decompose objects that are only
    defined by a synthesis routine. For example::

      from qiskit import QuantumCircuit
      from qiskit.quantum_info import Clifford
      from qiskit.transpiler.passes import Decompose

      cliff = Clifford(HGate())
      circuit = QuantumCircuit(1)
      circuit.append(cliff, [0])

      # Clifford has no .definition, it is only defined by synthesis
      nothing_happened = Decompose()(circuit)

      # this internally runs the HighLevelSynthesis pass to decompose the Clifford
      decomposed = Decompose(apply_synthesis=True)(circuit)

fixes:
  - |
    Fixed a bug in :meth:`.QuantumCircuit.decompose` where objects that could be synthesized
    with :class:`.HighLevelSynthesis` were first synthesized and then decomposed immediately
    (i.e., they were decomposed twice instead of once). This affected, e.g., :class:`.MCXGate`
    or :class:`.Clifford`, among others.
  - |
    Fixed a bug in :meth:`.QuantumCircuit.decompose`, where high-level objects without a definition
    were not decomposed if they were explicitly set via the ``"gates_to_decompose"`` argument.
    For example, previously the following did not perform a decomposition but now works as
    expected::

      from qiskit import QuantumCircuit
      from qiskit.quantum_info import Clifford
      from qiskit.transpiler.passes import Decompose

      cliff = Clifford(HGate())
      circuit = QuantumCircuit(1)
      circuit.append(cliff, [0])

      decomposed = Decompose(gates_to_decompose=["clifford"])(circuit)