error[E0191]: the value of the associated type `Output` in `Add` must be specified
  --> $DIR/issue-21950.rs:10:25
   |
LL |     type Output;
   |     ----------- `Output` defined here
...
LL |     let x = &10 as &dyn Add;
   |                         ^^^ help: specify the associated type: `Add<Output = Type>`

error[E0393]: the type parameter `Rhs` must be explicitly specified
  --> $DIR/issue-21950.rs:10:25
   |
LL | trait Add<Rhs=Self> {
   | ------------------- type parameter `Rhs` must be specified for this
...
LL |     let x = &10 as &dyn Add;
   |                         ^^^
   |
   = note: because of the default `Self` reference, type parameters must be specified on object types
help: set the type parameter to the desired type
   |
LL |     let x = &10 as &dyn Add<Rhs>;
   |                            +++++

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0191, E0393.
For more information about an error, try `rustc --explain E0191`.