Merge pull request #15095 from hvitved/dataflow/boolean-class

Data flow: Use `Boolean` class

shared/dataflow/codeql/dataflow/internal/DataFlowImpl.qll

@@ -6,6 +6,7 @@
 
 private import codeql.util.Unit
 private import codeql.util.Option
+private import codeql.util.Boolean
 private import codeql.dataflow.DataFlow
 
 module MakeImpl<InputSig Lang> {
@@ -1183,7 +1184,9 @@ module MakeImpl<InputSig Lang> {
           string toString();
         }
 
-        class Ap;
+        class Ap {
+          string toString();
+        }
 
         class ApNil extends Ap;
 
@@ -2432,9 +2435,7 @@ module MakeImpl<InputSig Lang> {
 
       class Typ = Unit;
 
-      class Ap extends boolean {
-        Ap() { this in [true, false] }
-      }
+      class Ap = Boolean;
 
       class ApNil extends Ap {
         ApNil() { this = false }

shared/util/codeql/util/Boolean.qll

@@ -5,6 +5,12 @@
  *
  * As opposed to `boolean`, this type does not require explicit binding.
  */
-class Boolean extends boolean {
+final class Boolean extends FinalBoolean {
   Boolean() { this = [true, false] }
+
+  /** Returns either "true" or "false". */
+  // reimplement to avoid explicit binding
+  string toString() { result = super.toString() }
 }
+
+final private class FinalBoolean = boolean;