error[E0119]: conflicting implementations of trait `Sweet` for type `std::boxed::Box<_>`:
  --> $DIR/coherence-overlap-issue-23516.rs:18:1
   |
17 | impl<T:Sugar> Sweet for T { }
   | ------------------------- first implementation here
18 | impl<U:Sugar> Sweet for Box<U> { }
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `std::boxed::Box<_>`
   |
   = note: downstream crates may implement trait `Sugar` for type `std::boxed::Box<_>`

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0119"