C++: Fix join-order problem in getNextSwitchCase

cpp/ql/lib/change-notes/2026-03-28-switch-stmt.md

@@ -0,0 +1,4 @@
+---
+category: feature
+---
+* A new predicate `getSwitchCase` was added to the `SwitchStmt` class, which yields the `n`th `case` statement from a `switch` statement.

cpp/ql/lib/semmle/code/cpp/stmts/Stmt.qll

@@ -1516,8 +1516,10 @@ class SwitchCase extends Stmt, @stmt_switch_case {
    * which has result `default:`, which has no result.
    */
   SwitchCase getNextSwitchCase() {
-    result.getSwitchStmt() = this.getSwitchStmt() and
-    result.getChildNum() = this.getChildNum() + 1
+    exists(SwitchStmt s, int n |
+      this = s.getSwitchCase(n) and
+      result = s.getSwitchCase(n + 1)
+    )
   }
 
   /**
@@ -1827,6 +1829,26 @@ class SwitchStmt extends ConditionalStmt, @stmt_switch {
    */
   SwitchCase getASwitchCase() { switch_case(underlyingElement(this), _, unresolveElement(result)) }
 
+  /**
+   * Gets the `n`th 'switch case' statement of this 'switch' statement, where
+   * `n` is 0-based.
+   *
+   * For example, for
+   * ```
+   * switch(i) {
+   *     case 1:
+   *     case 2:
+   *     break;
+   *     default:
+   *     break;
+   * }
+   * ```
+   * 0 yields `case 5:`, 1 yields `case 6:`, and 2 yields `default:`.
+   */
+  SwitchCase getSwitchCase(int n) {
+    switch_case(underlyingElement(this), n, unresolveElement(result))
+  }
+
   /**
    * Gets the 'default case' statement of this 'switch' statement,
    * if any.