error[E0658]: trait aliases are experimental
  --> $DIR/feature-gate-trait-alias.rs:1:1
   |
LL | trait Foo = Default;
   | ^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/41517
   = help: add `#![feature(trait_alias)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.