warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/specialization-unconstrained.rs:3:12
   |
LL | #![feature(specialization)]
   |            ^^^^^^^^^^^^^^
   |
   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
   = help: consider using `min_specialization` instead, which is more stable and complete
   = note: `#[warn(incomplete_features)]` on by default

error[E0284]: type annotations needed: cannot satisfy `<u32 as Default>::Id == ()`
  --> $DIR/specialization-unconstrained.rs:20:5
   |
LL |     test::<u32, ()>();
   |     ^^^^^^^^^^^^^^^ cannot satisfy `<u32 as Default>::Id == ()`
   |
note: required by a bound in `test`
  --> $DIR/specialization-unconstrained.rs:17:20
   |
LL | fn test<T: Default<Id = U>, U>() {}
   |                    ^^^^^^ required by this bound in `test`

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0284`.