Ruby: Add subclassing support to API Graphs

Given the code

    class A; end
    class B < A; end
    class C < A; end

You can find uses of B and C with the expression

    API::getTopLevelMember("A").getASubclass()

ruby/ql/lib/codeql/ruby/ApiGraphs.qll

@@ -254,10 +254,9 @@ module API {
      */
     pragma[nomagic]
     private predicate useRoot(string lbl, DataFlow::Node ref) {
-      exists(string name, ExprNodes::ConstantAccessCfgNode access, ConstantReadAccess read |
-        access = ref.asExpr() and
-        lbl = Label::member(read.getName()) and
-        read = access.getExpr()
+      exists(string name, ConstantReadAccess read |
+        read = ref.asExpr().getExpr() and
+        lbl = Label::member(read.getName())
       |
         name = resolveTopLevel(read)
         or
@@ -389,6 +388,17 @@ module API {
           useStep(lbl, node, ref)
         )
       )
+      or
+      // `pred` is a use of class A
+      // `succ` is a use of class B
+      // there exists a class declaration B < A
+      exists(ClassDeclaration c, DataFlow::Node a, DataFlow::Node b |
+        use(pred, a) and
+        use(succ, b) and
+        resolveConstant(b.asExpr().getExpr()) = resolveConstantWriteAccess(c) and
+        c.getSuperclassExpr() = a.asExpr().getExpr() and
+        lbl = Label::subclass()
+      )
     }
 
     /**

ruby/ql/lib/codeql/ruby/ast/internal/Module.qll

@@ -233,12 +233,17 @@ private module ResolveImpl {
 
   pragma[nomagic]
   private string resolveConstantReadAccessNonRec(ConstantReadAccess c, int priority) {
+    // ::B
     c.hasGlobalScope() and result = c.getName() and priority = 0
     or
+    // A::B
     exists(string name, string s | result = isDefinedConstantNonRec(s, name) |
       s = resolveConstantReadAccessScopeNonRec(c, priority, name)
     )
     or
+    // module A
+    //   B
+    // end
     exists(string name |
       exists(Namespace n, string qname |
         n = constantReadAccessEnclosingNameSpace(c, priority, name) and