C++: Reproduce a reported FP

Author: jbj

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

@@ -6,3 +6,4 @@
 | test.cpp:39:23:39:28 | ... == ... | This '==' operator has no effect. The assignment ('=') operator was probably intended. |
 | test.cpp:42:23:42:28 | ... == ... | This '==' operator has no effect. The assignment ('=') operator was probably intended. |
 | test.cpp:51:13:51:13 | call to operator== | This '==' operator has no effect. The assignment ('=') operator was probably intended. |
+| test.cpp:61:44:61:51 | ... == ... | This '==' operator has no effect. The assignment ('=') operator was probably intended. |

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 [FALSE POSITIVE]
+  return x1 == x2;
+}