missing a return *this in a copy-assignment

CppCoreGuidelines.md

@@ -21354,6 +21354,7 @@ Besides destructors and deallocation functions, common error-safety techniques r
     T& T::operator=(const T& other) {
         auto temp = other;
         swap(temp);
+        return *this;
     }
 
 (See also Item 56. ???)