C++: Support C11 _Generic expressions

cpp/downgrades/0fea0ee7026c7c3f7d6faef4df4bf67847b67d71/downgrades.ql

@@ -0,0 +1,32 @@
+/*
+ * Approach: replace conversion expressions of kind 389 (= @c11_generic) by
+ * conversion expressions of kind 12 (= @parexpr), i.e., a `ParenthesisExpr`,
+ * and drop the relation which its child expressions, which are just syntactic
+ * sugar. Parenthesis expressions are equally benign as C11 _Generic expressions,
+ * and behave similarly in the context of the IR.
+ */
+
+class Expr extends @expr {
+  string toString() { none() }
+}
+
+class Location extends @location {
+  string toString() { none() }
+}
+
+class ExprParent extends @exprparent {
+  string toString() { none() }
+}
+
+query predicate new_exprs(Expr expr, int new_kind, Location loc) {
+  exists(int kind | exprs(expr, kind, loc) | if kind = 389 then new_kind = 12 else new_kind = kind)
+}
+
+query predicate new_exprparents(Expr expr, int index, ExprParent expr_parent) {
+  exprparents(expr, index, expr_parent) and
+  (
+    not expr_parent instanceof @expr
+    or
+    exists(int kind | exprs(expr_parent.(Expr), kind, _) | kind != 389)
+  )
+}