error[E0393]: the type parameter `Rhs` must be explicitly specified
  --> $DIR/issue-21950.rs:5:18
   |
LL |               &dyn Add;
   |                    ^^^ help: set the type parameter to the desired type: `Add<Rhs>`
   | 
  ::: $SRC_DIR/libcore/ops/arith.rs:LL:COL
   |
LL | / pub trait Add<Rhs = Self> {
LL | |     /// The resulting type after applying the `+` operator.
LL | |     #[stable(feature = "rust1", since = "1.0.0")]
LL | |     type Output;
...  |
LL | |     fn add(self, rhs: Rhs) -> Self::Output;
LL | | }
   | |_- type parameter `Rhs` must be specified for this
   |
   = note: because of the default `Self` reference, type parameters must be specified on object types

error[E0191]: the value of the associated type `Output` (from trait `std::ops::Add`) must be specified
  --> $DIR/issue-21950.rs:5:18
   |
LL |             &dyn Add;
   |                  ^^^ help: specify the associated type: `Add<Output = Type>`

error: aborting due to 2 previous errors

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