error[E0592]: duplicate definitions with name `dummy`
  --> $DIR/coherence-overlap-issue-23516-inherent.rs:13:25
   |
LL | impl<T:Sugar> Cake<T> { fn dummy(&self) { } }
   |                         ^^^^^^^^^^^^^^^^^^^ duplicate definitions for `dummy`
...
LL | impl<U:Sugar> Cake<Box<U>> { fn dummy(&self) { } }
   |                              ------------------- other definition for `dummy`
   |
   = note: downstream crates may implement trait `Sugar` for type `std::boxed::Box<_>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0592`.