error[E0391]: cycle detected when computing type of `Foo::X`
  --> $DIR/cycle-trait-default-type-trait.rs:4:23
   |
LL | trait Foo<X = Box<dyn Foo>> {
   |                       ^^^
   |
   = note: ...which immediately requires computing type of `Foo::X` again
note: cycle used when checking that `Foo` is well-formed
  --> $DIR/cycle-trait-default-type-trait.rs:4:1
   |
LL | trait Foo<X = Box<dyn Foo>> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0391`.