suggest replacing 'this instanceof ...' in constructor with non-extending subtypes

ql/src/queries/style/suggestInstanceofExtension.ql

@@ -0,0 +1,47 @@
+/**
+ * @name Suggest using non-extending subtype relationships.
+ * @description Non-extending subtypes ("instanceof extensions") are generally preferrable to instanceof expressions in characteristic predicates.
+ * @kind problem
+ * @problem.severity warning
+ * @id ql/suggest-instanceof-extension
+ * @tags maintainability
+ * @precision medium
+ */
+
+import ql
+
+InstanceOf instanceofInCharPred(Class c) {
+  result = c.getCharPred().getBody()
+  or
+  exists(Conjunction conj |
+    conj = c.getCharPred().getBody() and
+    result = conj.getAnOperand()
+  )
+}
+
+predicate instanceofThisInCharPred(Class c, TypeExpr type) {
+  exists(InstanceOf instanceOf |
+    instanceOf = instanceofInCharPred(c) and
+    instanceOf.getExpr() instanceof ThisAccess and
+    type = instanceOf.getType()
+  )
+}
+
+predicate classWithInstanceofThis(Class c, TypeExpr type) {
+  instanceofThisInCharPred(c, type) and
+  exists(ClassPredicate classPred |
+    classPred = c.getAClassPredicate() and
+    exists(MemberCall call, InlineCast cast |
+      call.getEnclosingPredicate() = classPred and
+      cast = call.getBase() and
+      cast.getBase() instanceof ThisAccess and
+      cast.getTypeExpr().getResolvedType() = type.getResolvedType()
+    )
+  )
+}
+
+from Class c, TypeExpr type, string message
+where
+  classWithInstanceofThis(c, type) and
+  message = "consider defining $@ as non-extending subtype of $@"
+select c, message, c, c.getName(), type, type.getResolvedType().getName()