C++: add predicate to distinguish between array/field designators

cpp/ql/lib/semmle/code/cpp/exprs/Literal.qll

@@ -213,7 +213,27 @@ class ClassAggregateLiteral extends AggregateLiteral {
   Expr getFieldExpr(Field field, int position) {
     field = classType.getAField() and
     aggregate_field_init(underlyingElement(this), unresolveElement(result), unresolveElement(field),
-      position)
+      position, _)
+  }
+
+  /**
+   * Holds if the `position`-th initialization of `field` in this aggregate initializer
+   * uses a designator (e.g., `.x =`, `[42] =`) rather than a positional initializer.
+   *
+   * This can be used to distinguish explicitly designated initializations from
+   * implicit positional ones.
+   *
+   * For example, in the initializer:
+   * ```c
+   * struct S { int x, y; };
+   * struct S s = { .x = 1, 2 };
+   * ```
+   * - `.x = 1` is a designator init, therefore `isDesignatorInit(x, 0)` holds.
+   * - `2` is a positional init for `.y`, therefore `isDesignatorInit(y, 1)` does **not** hold.
+   */
+  predicate isDesignatorInit(Field field, int position) {
+    field = classType.getAField() and
+    aggregate_field_init(underlyingElement(this), _, unresolveElement(field), position, true)
   }
 
   /**
@@ -304,7 +324,24 @@ class ArrayOrVectorAggregateLiteral extends AggregateLiteral {
    * - `a.getElementExpr(0, 2)` gives `789`.
    */
   Expr getElementExpr(int elementIndex, int position) {
-    aggregate_array_init(underlyingElement(this), unresolveElement(result), elementIndex, position)
+    aggregate_array_init(underlyingElement(this), unresolveElement(result), elementIndex, position,
+      _)
+  }
+
+  /**
+   * Holds if the `position`-th initialization of the array element at `elementIndex`
+   * in this aggregate initializer uses a designator (e.g., `[0] = ...`) rather than
+   * an implicit positional initializer.
+   *
+   * For example, in:
+   * ```c
+   * int x[] = { [0] = 1, 2 };
+   * ```
+   * - `[0] = 1` is a designator init, therefore `isDesignatorInit(0, 0)` holds.
+   * - `2` is a positional init for `x[1]`, therefore `isDesignatorInit(1, 1)` does **not** hold.
+   */
+  predicate isDesignatorInit(int elementIndex, int position) {
+    aggregate_array_init(underlyingElement(this), _, elementIndex, position, true)
   }
 
   /**

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -2039,7 +2039,8 @@ aggregate_field_init(
     int aggregate: @aggregateliteral ref,
     int initializer: @expr ref,
     int field: @membervariable ref,
-    int position: int ref
+    int position: int ref,
+    boolean designated: boolean ref
 );
 
 /**
@@ -2051,7 +2052,8 @@ aggregate_array_init(
     int aggregate: @aggregateliteral ref,
     int initializer: @expr ref,
     int element_index: int ref,
-    int position: int ref
+    int position: int ref,
+    boolean designated: boolean ref
 );
 
 @ctorinit = @ctordirectinit