BlueprintCircuit subclasses rebuild out-of-order after _append

[jakelishman]

Environment

• Qiskit Terra version: 0.25.3
• Python version: 3.10
• Operating system: macOS

What is happening?

Subclasses of BlueprintCircuit do not behave correctly when the inner-loop _append method is used; the circuit is not rebuilt prior to accepting the append.

How can we reproduce the issue?

from qiskit.circuit.library import RealAmplitudes, XGate
from qiskit.circuit import CircuitInstruction, ClassicalRegister

qc = RealAmplitudes(num_qubits=2, reps=2)
qc.add_register(ClassicalRegister(2, "meas"))
qc._append(CircuitInstruction(XGate(), (qc.qubits[0],), ()))
qc.draw()

gives

        ┌───┐┌────────────────────────────────────────────────┐
   q_0: ┤ X ├┤0                                               ├
        └───┘│  RealAmplitudes(θ[0],θ[1],θ[2],θ[3],θ[4],θ[5]) │
   q_1: ─────┤1                                               ├
             └────────────────────────────────────────────────┘
meas: 2/═══════════════════════════════════════════════════════

whereas using QuantumCircuit.append gives the expected:

from qiskit.circuit.library import RealAmplitudes, XGate
from qiskit.circuit import CircuitInstruction, ClassicalRegister

qc = RealAmplitudes(num_qubits=2, reps=2)
qc.add_register(ClassicalRegister(2, "meas"))
qc.append(XGate(), [qc.qubits[0]])
qc.draw()

        ┌────────────────────────────────────────────────┐┌───┐
   q_0: ┤0                                               ├┤ X ├
        │  RealAmplitudes(θ[0],θ[1],θ[2],θ[3],θ[4],θ[5]) │└───┘
   q_1: ┤1                                               ├─────
        └────────────────────────────────────────────────┘
meas: 2/═══════════════════════════════════════════════════════

What should happen?

Inner _append should handle enforcing the build, not append.

Any suggestions?

No response