JS: Track types of classes in data flow

javascript/ql/lib/semmle/javascript/dataflow/internal/DataFlowPrivate.qll

@@ -558,15 +558,19 @@ DataFlowCallable nodeGetEnclosingCallable(Node node) {
 
 newtype TDataFlowType =
   TFunctionType(Function f) or
+  TInstanceType(DataFlow::ClassNode cls) or
   TAnyType()
 
 class DataFlowType extends TDataFlowType {
   string toDebugString() {
-    this instanceof TFunctionType and
     result =
       "TFunctionType(" + this.asFunction().toString() + ") at line " +
         this.asFunction().getLocation().getStartLine()
     or
+    result =
+      "TInstanceType(" + this.asInstanceOfClass().toString() + ") at line " +
+        this.asInstanceOfClass().getLocation().getStartLine()
+    or
     this instanceof TAnyType and result = "TAnyType"
   }
 
@@ -575,13 +579,25 @@ class DataFlowType extends TDataFlowType {
   }
 
   Function asFunction() { this = TFunctionType(result) }
+
+  DataFlow::ClassNode asInstanceOfClass() { this = TInstanceType(result) }
+}
+
+private predicate typeStrongerThan1(DataFlowType t1, DataFlowType t2) {
+  // 't1' is a subclass of 't2'
+  t1.asInstanceOfClass() = t2.asInstanceOfClass().getADirectSubClass()
 }
 
 /**
  * Holds if `t1` is strictly stronger than `t2`.
  */
 predicate typeStrongerThan(DataFlowType t1, DataFlowType t2) {
-  t1 instanceof TFunctionType and t2 = TAnyType()
+  typeStrongerThan1(t1, t2)
+  or
+  // Ensure all types are transitively stronger than 'any'
+  not typeStrongerThan1(t1, _) and
+  not t1 = TAnyType() and
+  t2 = TAnyType()
 }
 
 private DataFlowType getPreciseType(Node node) {
@@ -590,6 +606,9 @@ private DataFlowType getPreciseType(Node node) {
     result = TFunctionType(f)
   )
   or
+  result.asInstanceOfClass() =
+    unique(DataFlow::ClassNode cls | cls.getAnInstanceReference().getALocalUse() = node)
+  or
   result = getPreciseType(node.getImmediatePredecessor())
   or
   result = getPreciseType(node.(PostUpdateNode).getPreUpdateNode())
@@ -683,18 +702,27 @@ predicate neverSkipInPathGraph(Node node) {
 string ppReprType(DataFlowType t) { none() }
 
 pragma[inline]
-private predicate compatibleTypesNonSymRefl(DataFlowType t1, DataFlowType t2) {
+private predicate compatibleTypesWithAny(DataFlowType t1, DataFlowType t2) {
   t1 != TAnyType() and
   t2 = TAnyType()
 }
 
+pragma[nomagic]
+private predicate compatibleTypes1(DataFlowType t1, DataFlowType t2) {
+  t1.asInstanceOfClass().getADirectSubClass+() = t2.asInstanceOfClass()
+}
+
 pragma[inline]
 predicate compatibleTypes(DataFlowType t1, DataFlowType t2) {
   t1 = t2
   or
-  compatibleTypesNonSymRefl(t1, t2)
+  compatibleTypesWithAny(t1, t2)
   or
-  compatibleTypesNonSymRefl(t2, t1)
+  compatibleTypesWithAny(t2, t1)
+  or
+  compatibleTypes1(t1, t2)
+  or
+  compatibleTypes1(t2, t1)
 }
 
 predicate forceHighPrecision(Content c) { none() }

javascript/ql/test/library-tests/TripleDot/subclass.js

@@ -29,6 +29,6 @@ class Subclass3 extends Base {
         this.baseMethod("safe");
     }
     subclassMethod(x) {
-        sink(x); // $ SPURIOUS: hasValueFlow=sub1 hasValueFlow=sub2
+        sink(x);
     }
 }