Fix UnitaryGate.qasm() with unused qubits (Qiskit#8234)

The OpenQASM 2 exporter has a special case for providing `gate`
definitions, which was implemented purely for `UnitaryGate`.  This is
stateful per instruction, and does not generally behave well with the
rest of the exporter.  This commit fixes one misbehaviour: the way the
qubit operands for the definition was calculated was unreliable in both
order and number of qubits, if any of the qubits were unaffected by
the gate operation.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/extensions/unitary.py

@@ -191,22 +191,13 @@ def qasm(self):
             _qasm_escape_gate_name(self.label) if self.label else "unitary" + str(id(self))
         )
 
-        # map from gates in the definition to params in the method
-        bit_to_qasm = OrderedDict()
-        current_bit = 0
-
+        qubit_to_qasm = {bit: f"p{i}" for i, bit in enumerate(self.definition.qubits)}
         gates_def = ""
         for instruction in self.definition.data:
 
-            # add bits from this gate to the overall set of params
-            for bit in instruction.qubits + instruction.clbits:
-                if bit not in bit_to_qasm:
-                    bit_to_qasm[bit] = "p" + str(current_bit)
-                    current_bit += 1
-
             curr_gate = "\t{} {};\n".format(
                 instruction.operation.qasm(),
-                ",".join([bit_to_qasm[j] for j in instruction.qubits + instruction.clbits]),
+                ",".join(qubit_to_qasm[qubit] for qubit in instruction.qubits),
             )
             gates_def += curr_gate
 
@@ -215,7 +206,7 @@ def qasm(self):
             "gate "
             + self._qasm_name
             + " "
-            + ",".join(bit_to_qasm.values())
+            + ",".join(qubit_to_qasm[qubit] for qubit in self.definition.qubits)
             + " {\n"
             + gates_def
             + "}"

releasenotes/notes/fix-qasm2-identity-as-unitary-aa2feeb05707a597.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    The OpenQASM 2 exporter (:meth:`.QuantumCircuit.qasm`) will now correctly
+    define the qubit parameters for :class:`.UnitaryGate` operations that do
+    not affect all the qubits they are defined over.
+    Fixed `#8224 <https://github.com/Qiskit/qiskit-terra/issues/8224>`__.

test/python/circuit/test_unitary.py

@@ -281,6 +281,20 @@ def test_qasm_2q_unitary(self):
         )
         self.assertEqual(expected_qasm, qc.qasm())
 
+    def test_qasm_unitary_noop(self):
+        """Test that an identifier unitary can be converted to OpenQASM 2"""
+        qc = QuantumCircuit(QuantumRegister(3, "q0"))
+        qc.unitary(numpy.eye(8), qc.qubits, label="unitary_identity")
+        expected_qasm = (
+            "OPENQASM 2.0;\n"
+            'include "qelib1.inc";\n'
+            "gate unitary_identity p0,p1,p2 {\n"
+            "}\n"
+            "qreg q0[3];\n"
+            "unitary_identity q0[0],q0[1],q0[2];\n"
+        )
+        self.assertEqual(expected_qasm, qc.qasm())
+
     def test_unitary_decomposition(self):
         """Test decomposition for unitary gates over 2 qubits."""
         qc = QuantumCircuit(3)