From b751a8c3d25aa57f853bb74ded4e3472b4081d9f Mon Sep 17 00:00:00 2001
From: Jake Lishman <jake.lishman@ibm.com>
Date: Tue, 11 Feb 2025 19:06:11 +0000
Subject: [PATCH] Consider `unit` in `Delay` comparisons (#13816)

The `unit` field was previously ignored, allowing delays in `dt` units
to compare equal to those in `s`. This commit does not add additional
on-the-fly unit conversion to the comparison: if the user specified
durations in different time multiples, they may have had a reason to
consider them non-equal.  This could be revisiting after the new system
for duration handling lands (i.e. the new functionality for `stretch`
and other delayed scheduling).

(cherry picked from commit 32eae98a2282fdd056a84b12836b124e1726b843)
---
 qiskit/circuit/delay.py                       |  5 +++
 .../notes/delay-compare-b7ecb26b94ff0cd3.yaml |  5 +++
 test/python/circuit/test_delay.py             | 37 +++++++++++++++++++
 3 files changed, 47 insertions(+)
 create mode 100644 releasenotes/notes/delay-compare-b7ecb26b94ff0cd3.yaml

diff --git a/qiskit/circuit/delay.py b/qiskit/circuit/delay.py
index 2f0613acb932..96d7168a80dc 100644
--- a/qiskit/circuit/delay.py
+++ b/qiskit/circuit/delay.py
@@ -81,6 +81,11 @@ def to_matrix(self) -> np.ndarray:
         """
         return self.__array__(dtype=complex)
 
+    def __eq__(self, other):
+        return (
+            isinstance(other, Delay) and self.unit == other.unit and self._compare_parameters(other)
+        )
+
     def __repr__(self):
         """Return the official string representing the delay."""
         return f"{self.__class__.__name__}(duration={self.params[0]}[unit={self.unit}])"
diff --git a/releasenotes/notes/delay-compare-b7ecb26b94ff0cd3.yaml b/releasenotes/notes/delay-compare-b7ecb26b94ff0cd3.yaml
new file mode 100644
index 000000000000..583dc1752838
--- /dev/null
+++ b/releasenotes/notes/delay-compare-b7ecb26b94ff0cd3.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Comparisons of :class:`~.circuit.Delay` instructions, including within circuits, now require the
+    units to be equal as well as the duration value.
diff --git a/test/python/circuit/test_delay.py b/test/python/circuit/test_delay.py
index 4259c5604606..fb989789e97d 100644
--- a/test/python/circuit/test_delay.py
+++ b/test/python/circuit/test_delay.py
@@ -99,6 +99,43 @@ def test_to_matrix_return_identity_matrix(self):
         expected = np.array([[1, 0], [0, 1]], dtype=complex)
         self.assertTrue(np.array_equal(actual, expected))
 
+    def test_equality(self):
+        # At the time `__eq__` was specialised for `Delay`, the class was undergoing changes and
+        # moving to Rust, so we didn't also modify the Python-space semantics to declare equality
+        # between (say) 1000ms and 1s.  We could revisit that decision once the data model settles.
+        #
+        # This test then deliberately doesn't assert about mixed-scale comparisons, only comparisons
+        # between the same units, and 'dt' to absolute times.
+        def circuit_from(delay):
+            out = QuantumCircuit(1)
+            out.append(delay, [0], [])
+            return out
+
+        a = Parameter("a")
+        left_instructions, right_instructions = [], []
+        left_circuits, right_circuits = [], []
+        for unit in ("s", "ms", "us", "ns", "ps", "dt"):
+            for base in (left_instructions, right_instructions):
+                base.append(Delay(1, unit))
+                base.append(Delay(5.0, unit))
+                base.append(Delay(a, unit))
+            for base in (left_circuits, right_circuits):
+                base.append(circuit_from(Delay(1, unit)))
+                base.append(circuit_from(Delay(5.0, unit)))
+                base.append(circuit_from(Delay(a, unit)))
+        self.assertEqual(left_instructions, right_instructions)
+        self.assertEqual(left_circuits, right_circuits)
+
+        # We can't do all the non-equal tests in a single list comparison, since any single list
+        # failure would mask any spurious successes.
+        for unit in ("s", "ms", "us", "ns", "ps"):
+            self.assertNotEqual(Delay(2, unit), Delay(2, "dt"))
+            self.assertNotEqual(circuit_from(Delay(2, unit)), circuit_from(Delay(2, "dt")))
+            self.assertNotEqual(Delay(2, "dt"), Delay(2, unit))
+            self.assertNotEqual(circuit_from(Delay(2, "dt")), circuit_from(Delay(2, unit)))
+            self.assertNotEqual(Delay(a, unit), Delay(a, "dt"))
+            self.assertNotEqual(circuit_from(Delay(a, unit)), circuit_from(Delay(a, "dt")))
+
 
 class TestParameterizedDelay(QiskitTestCase):
     """Test delay instruction with parameterized duration."""