error[E0271]: type mismatch resolving `<impl Future<Output = [async output]> as Future>::Output == ()`
  --> $DIR/issue-89686.rs:7:17
   |
LL | type G<'a, T> = impl Future<Output = ()>;
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found `()`
...
LL |         async move { self.f().await }
   |                    ------------------ the expected `async` block
   |
  ::: $SRC_DIR/core/src/future/mod.rs:LL:COL
   |
LL | pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
   |                                           ------------------------------- the expected opaque type
   |
   = note: expected associated type `<impl Future<Output = [async output]> as Future>::Output`
                    found unit type `()`
   = help: consider constraining the associated type `<impl Future<Output = [async output]> as Future>::Output` to `()`
   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html

error[E0277]: the trait bound `T: Trait` is not satisfied
  --> $DIR/issue-89686.rs:7:17
   |
LL | type G<'a, T> = impl Future<Output = ()>;
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `T`
   |
help: consider restricting type parameter `T`
   |
LL | type G<'a, T: Trait> = impl Future<Output = ()>;
   |             +++++++

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0271, E0277.
For more information about an error, try `rustc --explain E0271`.