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:9:19
   |
LL | type DefaultFoo = Foo;
   |                   ^^^
   = note: ...which again requires computing type of `Foo::T`, completing the cycle
note: cycle used when checking that `Foo` is well-formed
  --> $DIR/issue-34373.rs:7:1
   |
LL | pub struct Foo<T = Box<Trait<DefaultFoo>>>;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = 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[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 2 previous errors

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