C++: Introduce ElementBase class

By extending this class, a class can define its own `getLocation`
predicate without participating in the dispatch hierarchy of
`getLocation` as defined on `Element`. Classes wanting to override their
location previously had to define `getURL` or `hasLocationInfo` instead
and rely on these predicates not being defined on future versions of
`Element`.

cpp/ql/src/AlertSuppression.ql

@@ -54,7 +54,11 @@ class SuppressionComment extends CppStyleComment {
 /**
  * The scope of an alert suppression comment.
  */
-class SuppressionScope extends SuppressionComment {
+class SuppressionScope extends ElementBase {
+  SuppressionScope() {
+    this instanceof SuppressionComment
+  }
+
   /**
   * Holds if this element is at the specified location.
   * The location spans column `startcolumn` of line `startline` to
@@ -63,7 +67,7 @@ class SuppressionScope extends SuppressionComment {
   * [LGTM locations](https://lgtm.com/help/ql/locations).
   */
   predicate hasLocationInfo(string filepath, int startline, int startcolumn, int endline, int endcolumn) {
-    this.covers(filepath, startline, startcolumn, endline, endcolumn)
+    this.(SuppressionComment).covers(filepath, startline, startcolumn, endline, endcolumn)
   }
 }
 

cpp/ql/src/Architecture/Refactoring Opportunities/ClassesWithManyFields.ql

@@ -46,8 +46,9 @@ predicate masterVde(VariableDeclarationEntry master, VariableDeclarationEntry vd
   exists(VariableDeclarationEntry previous | previousVde(previous, vde) and masterVde(master, previous))
 }
 
-class VariableDeclarationGroup extends VariableDeclarationEntry {
+class VariableDeclarationGroup extends ElementBase {
   VariableDeclarationGroup() {
+    this instanceof VariableDeclarationEntry and
     not previousVde(_, this)
   }
   Class getClass() {
@@ -62,7 +63,7 @@ class VariableDeclarationGroup extends VariableDeclarationEntry {
       masterVde(this, last) and
       this instanceof VariableDeclarationGroup and
       not previousVde(last, _) and
-      this.getLocation() = lstart and
+      exists(VariableDeclarationEntry vde | vde=this and vde instanceof VariableDeclarationEntry and vde.getLocation() = lstart) and
       last.getLocation() = lend and
       lstart.hasLocationInfo(path, startline, startcol, _, _) and
       lend.hasLocationInfo(path, _, _, endline, endcol)
@@ -78,7 +79,7 @@ class VariableDeclarationGroup extends VariableDeclarationEntry {
                                   name = vde.getName()))
              + " fields here"
     else
-      result = "declaration of " + this.getVariable().getName()
+      result = "declaration of " + this.(VariableDeclarationEntry).getVariable().getName()
   }
 }
 

cpp/ql/src/semmle/code/cpp/Element.qll

@@ -46,16 +46,29 @@ Element mkElement(@element e) {
 }
 
 /**
- * A C/C++ element. This class is the base class for all C/C++
- * elements, such as functions, classes, expressions, and so on.
+ * A C/C++ element with no member predicates other than `toString`. Not for
+ * general use. This class does not define a location, so classes wanting to
+ * change their location without affecting other classes can extend
+ * `ElementBase` instead of `Element` to create a new rootdef for `getURL`,
+ * `getLocation`, or `hasLocationInfo`.
  */
-class Element extends @element {
-  Element() {
+class ElementBase extends @element {
+  ElementBase() {
     this = resolveElement(_)
   }
 
   /** Gets a textual representation of this element. */
   string toString() { none() }
+}
+
+/**
+ * A C/C++ element. This class is the base class for all C/C++
+ * elements, such as functions, classes, expressions, and so on.
+ */
+class Element extends ElementBase {
+  Element() {
+    this = resolveElement(_)
+  }
 
   /** Gets the primary file where this element occurs. */
   File getFile() { result = this.getLocation().getFile() }

cpp/ql/src/semmle/code/cpp/controlflow/ControlFlowGraph.qll

@@ -146,12 +146,12 @@ private predicate loopConditionAlwaysUponEntry(ControlFlowNode loop, Expr condit
 /**
  * An element that is convertible to `ControlFlowNode`. This class is similar
  * to `ControlFlowNode` except that is has no member predicates apart from
- * those inherited from `Locatable`.
+ * `toString`.
  *
  * This class can be used as base class for classes that want to inherit the
  * extent of `ControlFlowNode` without inheriting its public member predicates.
  */
-class ControlFlowNodeBase extends Locatable, @cfgnode {
+class ControlFlowNodeBase extends ElementBase, @cfgnode {
 }
 
 predicate truecond_base(ControlFlowNodeBase n1, ControlFlowNodeBase n2) {

cpp/ql/src/semmle/code/cpp/controlflow/DefinitionsAndUses.qll

@@ -88,7 +88,7 @@ class DefOrUse extends ControlFlowNodeBase {
     // Uninstantiated templates are purely syntax, and only on instantiation
     // will they be complete with information about types, conversions, call
     // targets, etc.
-    not this.isFromUninstantiatedTemplate(_)
+    not this.(ControlFlowNode).isFromUninstantiatedTemplate(_)
   }
 
   /**

cpp/ql/src/semmle/code/cpp/controlflow/SSA.qll

@@ -70,6 +70,10 @@ class SsaDefinition extends ControlFlowNodeBase {
         exists(StandardSSA x | x.phi_node(v, (BasicBlock)this))
     }
 
+    Location getLocation() {
+        result = this.(ControlFlowNode).getLocation()
+    }
+
     /** Holds if the SSA variable `(this, p)` is defined by parameter `p`. */
     predicate definedByParameter(Parameter p) {
         this = p.getFunction().getEntryPoint()

cpp/ql/src/semmle/code/cpp/rangeanalysis/RangeSSA.qll

@@ -113,6 +113,10 @@ class RangeSsaDefinition extends ControlFlowNodeBase {
         guard_defn(v, guard, this, branch)
     }
 
+    Location getLocation() {
+        result = this.(ControlFlowNode).getLocation()
+    }
+
     /** Whether this definition is from a parameter */
     predicate definedByParameter(Parameter p) {
         this = p.getFunction().getEntryPoint()