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 this bound in `S5`
LL | 
LL | impl<X: ?Sized> T3<X> for S5<X> {
   |      -          ^^^^^ doesn't have a size known at compile-time
   |      |
   |      this type parameter needs to be `std::marker::Sized`
   |
   = 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: you could relax the implicit `Sized` bound on `Y` if it were used through indirection like `&Y` or `Box<Y>`
  --> $DIR/unsized-trait-impl-self-type.rs:8:11
   |
LL | struct S5<Y>(Y);
   |           ^  - ...if indirection was used here: `Box<Y>`
   |           |
   |           this could be changed to `Y: ?Sized`...

error: aborting due to previous error

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