error[E0277]: the size for values of type `X` cannot be known at compilation time
  --> $DIR/unsized-trait-impl-self-type.rs:10:17
   |
LL | struct S5<Y>(Y);
   | ---------------- required by `S5`
LL | 
LL | impl<X: ?Sized> T3<X> for S5<X> {
   |                 ^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `X`
   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = help: consider adding a `where X: std::marker::Sized` bound

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.