Merge pull request #2391 from jbj/CompareWhereAssignMeant-decltype

C++: Fix FP for expression SFINAE with decltype

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

@@ -15,7 +15,10 @@ import cpp
 
 from ExprInVoidContext op
 where
-  op instanceof EQExpr
-  or
-  op.(FunctionCall).getTarget().hasName("operator==")
+  not op.isUnevaluated() and
+  (
+    op instanceof EQExpr
+    or
+    op.(FunctionCall).getTarget().hasName("operator==")
+  )
 select op, "This '==' operator has no effect. The assignment ('=') operator was probably intended."

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

@@ -86,6 +86,7 @@ where
   not peivc.isFromTemplateInstantiation(_) and
   parent = peivc.getParent() and
   not parent.isInMacroExpansion() and
+  not peivc.isUnevaluated() and
   not parent instanceof PureExprInVoidContext and
   not peivc.getEnclosingFunction().isCompilerGenerated() and
   not peivc.getType() instanceof UnknownType and

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

@@ -55,3 +55,9 @@ void f(void) {
     }
 }
 
+// This pattern is used to emulate C++20 concepts in a way that's very light on
+// template syntax.
+template<typename T1, typename T2>
+auto sfinaeTrick(T1 x1, T2 x2) -> decltype(x1 == x2, bool()) { // GOOD
+  return x1 == x2;
+}