error[E0391]: cycle detected when computing type of `Foo::T`
  --> $DIR/issue-34373.rs:7:30
   |
LL | pub struct Foo<T = Box<Trait<DefaultFoo>>>;
   |                              ^^^^^^^^^^
   |
note: ...which requires expanding type alias `DefaultFoo`...
  --> $DIR/issue-34373.rs:10:19
   |
LL | type DefaultFoo = Foo;
   |                   ^^^
   = note: ...which again requires computing type of `Foo::T`, completing the cycle
note: cycle used when collecting item types in top-level module
  --> $DIR/issue-34373.rs:1:1
   |
LL | / #![allow(warnings)]
LL | |
LL | | trait Trait<T> {
LL | |     fn foo(_: T) {}
...  |
LL | | fn main() {
LL | | }
   | |_^
   = 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[E0038]: the trait `Trait` cannot be made into an object
  --> $DIR/issue-34373.rs:7:24
   |
LL | pub struct Foo<T = Box<Trait<DefaultFoo>>>;
   |                        ^^^^^^^^^^^^^^^^^ `Trait` 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-34373.rs:4:8
   |
LL | trait Trait<T> {
   |       ----- this trait cannot be made into an object...
LL |     fn foo(_: T) {}
   |        ^^^ ...because associated function `foo` has no `self` parameter
help: consider turning `foo` into a method by giving it a `&self` argument
   |
LL |     fn foo(&self, _: T) {}
   |            ++++++
help: alternatively, consider constraining `foo` so it does not apply to trait objects
   |
LL |     fn foo(_: T) where Self: Sized {}
   |                  +++++++++++++++++

error[E0392]: type parameter `T` is never used
  --> $DIR/issue-34373.rs:7:16
   |
LL | pub struct Foo<T = Box<Trait<DefaultFoo>>>;
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^ unused type parameter
   |
   = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0038, E0391, E0392.
For more information about an error, try `rustc --explain E0038`.