Fix #3412

This commits fixes an issue that disallowed the usage of
`BoxableExpression` as order clause due to a malformed trait impl in diesel.

diesel/src/query_builder/select_statement/boxed.rs

@@ -389,9 +389,12 @@ where
     }
 }
 
-impl<'a, ST, QS, DB, Order, GB> OrderDsl<Order> for BoxedSelectStatement<'a, ST, QS, DB, GB>
+// no impls for `NoFromClause` here because order is not really supported there yet
+impl<'a, ST, QS, DB, Order, GB> OrderDsl<Order>
+    for BoxedSelectStatement<'a, ST, FromClause<QS>, DB, GB>
 where
     DB: Backend,
+    QS: QuerySource,
     Order: QueryFragment<DB> + AppearsOnTable<QS> + Send + 'a,
 {
     type Output = Self;
@@ -402,9 +405,11 @@ where
     }
 }
 
-impl<'a, ST, QS, DB, Order, GB> ThenOrderDsl<Order> for BoxedSelectStatement<'a, ST, QS, DB, GB>
+impl<'a, ST, QS, DB, Order, GB> ThenOrderDsl<Order>
+    for BoxedSelectStatement<'a, ST, FromClause<QS>, DB, GB>
 where
     DB: Backend + 'a,
+    QS: QuerySource,
     Order: QueryFragment<DB> + AppearsOnTable<QS> + Send + 'a,
 {
     type Output = Self;

diesel/src/query_builder/select_statement/dsl_impls.rs

@@ -261,15 +261,18 @@ where
     }
 }
 
+// no impls for `NoFromClause` here because order is not really supported there yet
 impl<ST, F, S, D, W, O, LOf, G, H, LC, Expr> OrderDsl<Expr>
-    for SelectStatement<F, S, D, W, O, LOf, G, H, LC>
+    for SelectStatement<FromClause<F>, S, D, W, O, LOf, G, H, LC>
 where
+    F: QuerySource,
     Expr: AppearsOnTable<F>,
     Self: SelectQuery<SqlType = ST>,
-    SelectStatement<F, S, D, W, OrderClause<Expr>, LOf, G, H, LC>: SelectQuery<SqlType = ST>,
+    SelectStatement<FromClause<F>, S, D, W, OrderClause<Expr>, LOf, G, H, LC>:
+        SelectQuery<SqlType = ST>,
     OrderClause<Expr>: ValidOrderingForDistinct<D>,
 {
-    type Output = SelectStatement<F, S, D, W, OrderClause<Expr>, LOf, G, H, LC>;
+    type Output = SelectStatement<FromClause<F>, S, D, W, OrderClause<Expr>, LOf, G, H, LC>;
 
     fn order(self, expr: Expr) -> Self::Output {
         let order = OrderClause(expr);
@@ -288,11 +291,12 @@ where
 }
 
 impl<F, S, D, W, O, LOf, G, H, LC, Expr> ThenOrderDsl<Expr>
-    for SelectStatement<F, S, D, W, OrderClause<O>, LOf, G, H, LC>
+    for SelectStatement<FromClause<F>, S, D, W, OrderClause<O>, LOf, G, H, LC>
 where
+    F: QuerySource,
     Expr: AppearsOnTable<F>,
 {
-    type Output = SelectStatement<F, S, D, W, OrderClause<(O, Expr)>, LOf, G, H, LC>;
+    type Output = SelectStatement<FromClause<F>, S, D, W, OrderClause<(O, Expr)>, LOf, G, H, LC>;
 
     fn then_order_by(self, expr: Expr) -> Self::Output {
         SelectStatement::new(

diesel_tests/tests/order.rs

@@ -78,3 +78,54 @@ fn order_by_descending_column() {
     let data: Vec<_> = users.order(name.desc()).load(conn).unwrap();
     assert_eq!(expected_data, data);
 }
+
+// regression test for #3412
+#[test]
+fn dynamic_order() {
+    use crate::schema::users;
+    use diesel::expression::expression_types::NotSelectable;
+
+    let conn = &mut connection_with_sean_and_tess_in_users_table();
+    let expected = &["Tess", "Sean"] as &[_];
+
+    let order_field: Box<dyn BoxableExpression<users::table, _, SqlType = NotSelectable>> =
+        Box::new(users::id.desc());
+
+    let result = users::table
+        .select(users::name)
+        .order(order_field)
+        .load::<String>(conn)
+        .unwrap();
+    assert_eq!(expected, &result);
+
+    let order_field: Box<dyn BoxableExpression<users::table, _, SqlType = NotSelectable>> =
+        Box::new(users::id.desc());
+
+    let result = users::table
+        .select(users::name)
+        .then_order_by(order_field)
+        .load::<String>(conn)
+        .unwrap();
+    assert_eq!(expected, &result);
+
+    let order_field: Box<dyn BoxableExpression<users::table, _, SqlType = NotSelectable>> =
+        Box::new(users::id.desc());
+    let result = users::table
+        .select(users::name)
+        .into_boxed()
+        .order(order_field)
+        .load::<String>(conn)
+        .unwrap();
+    assert_eq!(expected, &result);
+
+    let order_field: Box<dyn BoxableExpression<users::table, _, SqlType = NotSelectable>> =
+        Box::new(users::id.desc());
+
+    let result = users::table
+        .select(users::name)
+        .into_boxed()
+        .then_order_by(order_field)
+        .load::<String>(conn)
+        .unwrap();
+    assert_eq!(expected, &result);
+}