C++: Exclude operator= in ExprHasNoEffect

Author: jbj

cpp/ql/src/Likely Bugs/Likely Typos/ExprHasNoEffect.ql

@@ -97,6 +97,10 @@ where // EQExprs are covered by CompareWhereAssignMeant.ql
       not peivc instanceof EQExpr and
       // as is operator==
       not peivc.(FunctionCall).getTarget().hasName("operator==") and
+      // An assignment operator may have no side effects in its current
+      // implementation, but we should not advise callers to rely on this. That
+      // would break encapsulation.
+      not peivc.(FunctionCall).getTarget().hasName("operator=") and
       not accessInInitOfForStmt(peivc) and
       not peivc.isCompilerGenerated() and
       not exists(Macro m | peivc = m.getAnInvocation().getAnExpandedElement()) and

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/ExprHasNoEffect.expected

@@ -27,9 +27,6 @@
 | test.c:27:9:27:10 | 33 | This expression has no effect. | test.c:27:9:27:10 | 33 |  |
 | test.cpp:24:3:24:3 | call to operator++ | This expression has no effect (because $@ has no external side effects). | test.cpp:9:14:9:23 | operator++ | operator++ |
 | test.cpp:25:3:25:3 | call to operator++ | This expression has no effect (because $@ has no external side effects). | test.cpp:9:14:9:23 | operator++ | operator++ |
-| test.cpp:62:5:62:5 | call to operator= | This expression has no effect (because $@ has no external side effects). | test.cpp:47:14:47:22 | operator= | operator= |
-| test.cpp:65:5:65:5 | call to operator= | This expression has no effect (because $@ has no external side effects). | test.cpp:55:7:55:7 | operator= | operator= |
-| test.cpp:91:5:91:19 | call to operator= | This expression has no effect (because $@ has no external side effects). | test.cpp:77:9:77:17 | operator= | operator= |
 | volatile.c:9:5:9:5 | c | This expression has no effect. | volatile.c:9:5:9:5 | c |  |
 | volatile.c:12:5:12:9 | access to array | This expression has no effect. | volatile.c:12:5:12:9 | access to array |  |
 | volatile.c:16:5:16:7 | * ... | This expression has no effect. | volatile.c:16:5:16:7 | * ... |  |

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/test.cpp

@@ -88,7 +88,7 @@ struct Derived : Base {
     }
 
     // In case base class has data, now or in the future, copy that first.
-    Base::operator=(rhs); // GOOD [FALSE POSITIVE]
+    Base::operator=(rhs); // GOOD
 
     this->m_x = rhs.m_x;
     return *this;