Add inverse method to ECRGate

qiskit/circuit/library/standard_gates/ecr.py

@@ -103,6 +103,10 @@ def _define(self):
 
         self.definition = qc
 
+    def inverse(self):
+        """Return inverse ECR gate (itself)."""
+        return ECRGate()  # self-inverse
+
     def to_matrix(self):
         """Return a numpy.array for the ECR gate."""
         return (

releasenotes/notes/add-inverse-ecr-e03720252a0c9c1e.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    The method :meth:`.ECRGate.inverse` now returns another :class:`.ECRGate` instance
+    rather than a custom gate, since it is self inverse.