[Test] Add a test case

Author: theblixguy

test/decl/ext/extensions.swift

@@ -124,3 +124,23 @@ struct WrapperContext {
     static let propUsingMember = originalValue
   }
 }
+
+// SR-11298
+
+protocol SR_11298_P {}
+
+class SR_11298_C: SR_11298_P {
+  var property: String = ""
+}
+
+// Self: SR_11298_C requirement constrains this extension to SR_11298C and its subclasses.
+// Since this implies a class constraint, the setter should be implicitly nonmutating.
+extension SR_11298_P where Self: SR_11298_C {
+  var wrappingProperty: String {
+    get { return property }
+    set { property = newValue }
+  }
+}
+
+let instance = SR_11298_C()
+instance.wrappingProperty = "" // Okay