Repository: Qiskit/qiskit

Issue:
```Gate.control``` method throwing error
### Environment

- **Qiskit Terra version**: 0.24.0
- **Python version**: Python 3.9.6

### What is happening?

Given a quantum circuit object, we execute the ```to_gate()``` method and attempt to call the ```Gate.control()``` method with appropriate parameters and sometimes observe the following error: ```*** UnboundLocalError: local variable 'mat2'``` referenced before assignment

### How can we reproduce the issue?

``` 
from qiskit.extensions import UnitaryGate
from qiskit import QuantumCircuit, QuantumRegister

# initialize a unitary matrix

A = np.array([[ 0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  1.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  1.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j, -1.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
        -1.+0.j],
       [ 1.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  1.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j, -1.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j,  0.+0.j, -1.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j]])

gate = UnitaryGate(A)
qc = QuantumCircuit(3)
qc.append(gate,[0,1,2])

# This happens in a a different function than the operations above

CU = qc.to_gate().control(6,28)
```

### What should happen?

I would assume that the error is not thrown

### Any suggestions?

The error appears to come from the file ```qsd.py```, and in particular because the list ```ind2q``` (line 234) has only one element. The for loop that starts on line 242 is thus never executed and the the variable ```mat2``` is never initialized. 


Discussion:
I believe this is a duplicate of #10036, so the fix should have been released in Terra 0.24.1.  Could you try upgrading and see if the problem persists?
Oh no, this still seems to be present on `main` despite #10126.  @ewinston, please can you look at this?
@jakelishman @ewinston  I see the lines of code that are meant to fix this (from 2 months ago). Lines 246 and 247 are:

```
if not ind2q:
        return ccirc
```

I could be wrong but it also seems like if the list ```ind2q``` has just one element, the ```for``` loop that comes next will not be executed and the variable ```mat2``` will never be initialized, thus throwing the same error.

Resolving patch:
diff --git a/qiskit/quantum_info/synthesis/qsd.py b/qiskit/quantum_info/synthesis/qsd.py
--- a/qiskit/quantum_info/synthesis/qsd.py
+++ b/qiskit/quantum_info/synthesis/qsd.py
@@ -243,7 +243,11 @@ def _apply_a2(circ):
     for i, instruction in enumerate(ccirc.data):
         if instruction.operation.name == "qsd2q":
             ind2q.append(i)
-    if not ind2q:
+    if len(ind2q) == 0:
+        return ccirc
+    elif len(ind2q) == 1:
+        # No neighbors to merge diagonal into; revert name
+        ccirc.data[ind2q[0]].operation.name = "Unitary"
         return ccirc
     # rolling over diagonals
     ind2 = None  # lint


Test patch:
diff --git a/test/python/quantum_info/test_synthesis.py b/test/python/quantum_info/test_synthesis.py
--- a/test/python/quantum_info/test_synthesis.py
+++ b/test/python/quantum_info/test_synthesis.py
@@ -1548,6 +1548,105 @@ def test_opt_a1a2(self, nqubits):
         elif nqubits == 2:
             self.assertLessEqual(ccirc.count_ops().get("cx", 0), 3)
 
+    def test_a2_opt_single_2q(self):
+        """
+        Test a2_opt when a unitary causes a single final 2-qubit unitary for which this optimization
+        won't help. This came up in issue 10787.
+        """
+        # this somewhat unique signed permutation matrix seems to cause the issue
+        mat = np.array(
+            [
+                [
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    1.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                ],
+                [
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    1.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                ],
+                [
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    -1.0 + 0.0j,
+                    0.0 + 0.0j,
+                ],
+                [
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    -1.0 + 0.0j,
+                ],
+                [
+                    1.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                ],
+                [
+                    0.0 + 0.0j,
+                    1.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                ],
+                [
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    -1.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                ],
+                [
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    -1.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                    0.0 + 0.0j,
+                ],
+            ]
+        )
+
+        gate = UnitaryGate(mat)
+        qc = QuantumCircuit(3)
+        qc.append(gate, range(3))
+        try:
+            qc.to_gate().control(1)
+        except UnboundLocalError as uerr:
+            self.fail(str(uerr))
+
 
 class TestTwoQubitDecomposeUpToDiagonal(QiskitTestCase):
     """test TwoQubitDecomposeUpToDiagonal class"""