error[E0261]: use of undeclared lifetime name `'a`
  --> $DIR/issue-67510.rs:5:21
   |
LL | fn f(x: Box<dyn X<Y<'a> = &'a ()>>) {}
   |                     ^^ undeclared lifetime
   |
   = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider making the bound lifetime-generic with a new `'a` lifetime
   |
LL | fn f(x: Box<dyn for<'a> X<Y<'a> = &'a ()>>) {}
   |                 +++++++
help: consider introducing lifetime `'a` here
   |
LL | fn f<'a>(x: Box<dyn X<Y<'a> = &'a ()>>) {}
   |     ++++

error[E0261]: use of undeclared lifetime name `'a`
  --> $DIR/issue-67510.rs:5:28
   |
LL | fn f(x: Box<dyn X<Y<'a> = &'a ()>>) {}
   |                            ^^ undeclared lifetime
   |
help: consider making the bound lifetime-generic with a new `'a` lifetime
   |
LL | fn f(x: Box<dyn for<'a> X<Y<'a> = &'a ()>>) {}
   |                 +++++++
help: consider introducing lifetime `'a` here
   |
LL | fn f<'a>(x: Box<dyn X<Y<'a> = &'a ()>>) {}
   |     ++++

error[E0038]: the trait `X` cannot be made into an object
  --> $DIR/issue-67510.rs:5:13
   |
LL | fn f(x: Box<dyn X<Y<'a> = &'a ()>>) {}
   |             ^^^^^^^^^^^^^^^^^^^^^ `X` cannot be made into an object
   |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> $DIR/issue-67510.rs:2:10
   |
LL | trait X {
   |       - this trait cannot be made into an object...
LL |     type Y<'a>;
   |          ^ ...because it contains the generic associated type `Y`
   = help: consider moving `Y` to another trait

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0038, E0261.
For more information about an error, try `rustc --explain E0038`.