warning: return type notation is experimental
  --> $DIR/feature-gate-return_type_notation.rs:17:17
   |
LL | fn foo<T: Trait<m(): Send>>() {}
   |                 ^^^^^^^^^
   |
   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
   = help: add `#![feature(return_type_notation)]` to the crate attributes to enable
   = warning: unstable syntax can change at any point in the future, causing a hard error!
   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>

warning: 1 warning emitted