error[E0277]: the size for values of type `[()]` cannot be known at compilation time
  --> $DIR/issue-61631-default-type-param-can-reference-self-in-trait.rs:19:6
   |
LL | trait Tsized<P: Sized = [Self]> {}
   |              - required by this bound in `Tsized`
LL | 
LL | impl Tsized for () {}
   |      ^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `[()]`

error: aborting due to previous error

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