Fix SetLayout to error with invalid int list input (#11653)

* Fix SetLayout to error with invalid int list input

This commit fixes an issue in the SetLayout transpiler pass when an
integer list is specified nothing was checking that the integer list
contained unique entries. Previously if a list containing duplciate
entries was passed in this would result in an invalid Layout object
being set on the property set. This would cause a cryptic error message
by later transpiler passes that would try and use that invalid Layout.
This commit fixes this by detecting if there are duplicate entries in
the int list and raising an exception directly.

* Use new exception class in test

qiskit/transpiler/__init__.py

@@ -1248,6 +1248,7 @@
 .. autoexception:: CouplingError
 .. autoexception:: LayoutError
 .. autoexception:: CircuitTooWideForTarget
+.. autoexception:: InvalidLayoutError
 
 """
 
@@ -1266,6 +1267,7 @@
     CouplingError,
     LayoutError,
     CircuitTooWideForTarget,
+    InvalidLayoutError,
 )
 from .basepasses import AnalysisPass, TransformationPass
 from .coupling import CouplingMap

qiskit/transpiler/exceptions.py

@@ -53,3 +53,7 @@ def __str__(self):
 
 class CircuitTooWideForTarget(TranspilerError):
     """Error raised if the circuit is too wide for the target."""
+
+
+class InvalidLayoutError(TranspilerError):
+    """Error raised when a user provided layout is invalid."""

qiskit/transpiler/passes/layout/set_layout.py

@@ -11,7 +11,8 @@
 # that they have been altered from the originals.
 
 """Set the ``layout`` property to the given layout."""
-from qiskit.transpiler import Layout, TranspilerError
+from qiskit.transpiler import Layout
+from qiskit.transpiler.exceptions import InvalidLayoutError
 from qiskit.transpiler.basepasses import AnalysisPass
 
 
@@ -47,17 +48,21 @@ def run(self, dag):
         """
         if isinstance(self.layout, list):
             if len(self.layout) != len(dag.qubits):
-                raise TranspilerError(
+                raise InvalidLayoutError(
                     "The length of the layout is different than the size of the "
                     f"circuit: {len(self.layout)} <> {len(dag.qubits)}"
                 )
+            if len(set(self.layout)) != len(self.layout):
+                raise InvalidLayoutError(
+                    f"The provided layout {self.layout} contains duplicate qubits"
+                )
             layout = Layout({phys: dag.qubits[i] for i, phys in enumerate(self.layout)})
         elif isinstance(self.layout, Layout):
             layout = self.layout.copy()
         elif self.layout is None:
             layout = None
         else:
-            raise TranspilerError(
+            raise InvalidLayoutError(
                 f"SetLayout was intialized with the layout type: {type(self.layout)}"
             )
         self.property_set["layout"] = layout

releasenotes/notes/add-invalid-layout-error-d0d64086748d4b54.yaml

@@ -0,0 +1,13 @@
+---
+features:
+  - |
+    Added a new exception class :class:`.InvalidLayoutError` that is a :class:`.TranspilerError`
+    subclass which is raised when a user provided layout is invalid (mismatched size, duplicate
+    qubits, etc).
+fixes:
+  - |
+    Fixed an issue with the :class:`.SetLayout` transpiler pass where an invalid integer list input
+    that contained duplicate entries which would result in an invalid :class:`.Layout` being
+    generated and subsequent transpiler passes would fail with a cryptic error. This is now caught
+    when :meth:`.SetLayout.run` is called an :class:`.InvalidLayoutError` error will be raised
+    indicating there are duplicate entries in the integer list.

test/python/transpiler/test_setlayout.py

@@ -18,7 +18,7 @@
 from qiskit.transpiler import CouplingMap, Layout
 from qiskit.transpiler.passes import SetLayout, ApplyLayout, FullAncillaAllocation
 from qiskit.test import QiskitTestCase
-from qiskit.transpiler import PassManager, TranspilerError
+from qiskit.transpiler import PassManager, TranspilerError, InvalidLayoutError
 
 
 class TestSetLayout(QiskitTestCase):
@@ -109,6 +109,13 @@ def test_raise_when_layout_len_does_not_match(self):
         with self.assertRaises(TranspilerError):
             pass_manager.run(circuit)
 
+    def test_raise_if_int_list_layout_contains_duplicates(self):
+        """Test the error is raised if the specified intlist contains duplicates"""
+        circuit = QuantumCircuit(4)
+        layout_pass = SetLayout([0, 1, 1, 2])
+        with self.assertRaises(InvalidLayoutError):
+            layout_pass(circuit)
+
 
 if __name__ == "__main__":
     unittest.main()