Inconsistency with “at least one non-builtin trait is required for an object type”

[bluss]

Auto traits are alone allowed as object types in some cases and not others.

The following two lines are fine: (Using Send as an object type)

type Y = Send;

fn main() {
    let x = &1 as &Send;
}

The following line has an error:

type Z = for<'x> Send;  //~ error[E0224]: at least one non-builtin trait is required for an object type

The second error seems to speak against the first two examples.

Tested using rustc 1.14.0-nightly (3f4408347 2016-10-27)

[Mark-Simulacrum]

The latter compiles fine today; I guess this should get a test.

warning: type alias is never used: `Z`
 --> test.rs:1:1
  |
1 | type Z = for<'x> Send;
  | ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default