C++: Add upgrade and downgrade scripts

cpp/downgrades/9a7c3c14c1076f64b871719117a558733d987b48/decltypes.ql

@@ -0,0 +1,11 @@
+class Type extends @type {
+  string toString() { none() }
+}
+
+class Expr extends @expr {
+  string toString() { none() }
+}
+
+from Type decltype, Expr expr, Type basetype, boolean parentheses
+where decltypes(decltype, expr, _, basetype, parentheses)
+select decltype, expr, basetype, parentheses

cpp/downgrades/9a7c3c14c1076f64b871719117a558733d987b48/derivedtypes.ql

@@ -0,0 +1,19 @@
+class Type extends @type {
+  string toString() { none() }
+}
+
+predicate derivedType(Type type, string name, int kind, Type type_id) {
+  derivedtypes(type, name, kind, type_id)
+}
+
+predicate typeTransformation(Type type, string name, int kind, Type type_id) {
+  type_operators(type, _, _, type_id) and
+  name = "" and
+  kind = 3 // @type_with_specifiers
+}
+
+from Type type, string name, int kind, Type type_id
+where
+  derivedType(type, name, kind, type_id) or
+  typeTransformation(type, name, kind, type_id)
+select type, name, kind, type_id

cpp/downgrades/9a7c3c14c1076f64b871719117a558733d987b48/upgrade.properties

@@ -0,0 +1,5 @@
+description: Support C23 typeof and typeof_unqual
+compatibility: backwards
+decltypes.rel: run decltypes.qlo
+derivedtypes.rel: run derivedtypes.qlo
+type_operators.rel: delete

cpp/ql/lib/upgrades/2e2d805ef93d060b813403cb9b51dc72455a4c68/decltypes.ql

@@ -0,0 +1,11 @@
+class Type extends @type {
+  string toString() { none() }
+}
+
+class Expr extends @expr {
+  string toString() { none() }
+}
+
+from Type decltype, Expr expr, Type basetype, boolean parentheses
+where decltypes(decltype, expr, basetype, parentheses)
+select decltype, expr, 0, basetype, parentheses

cpp/ql/lib/upgrades/2e2d805ef93d060b813403cb9b51dc72455a4c68/upgrade.properties

@@ -0,0 +1,3 @@
+description: Support C23 typeof and typeof_unqual
+compatibility: partial
+decltypes.rel: run decltypes.qlo