error[E0119]: conflicting implementations of trait `Trait<Bar, _>`
  --> $DIR/issue-84660-unsoundness.rs:31:1
   |
LL | impl<In, Out> Trait<Bar, In> for Out {
   | ------------------------------------ first implementation here
...
LL | impl<In, Out> Trait<(), In> for Out {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

error: item does not constrain `Bar::{opaque#0}`
  --> $DIR/issue-84660-unsoundness.rs:24:8
   |
LL |     fn convert(_i: In) -> Self::Out {
   |        ^^^^^^^
   |
   = note: consider removing `#[define_opaque]` or adding an empty `#[define_opaque()]`
note: this opaque type is supposed to be constrained
  --> $DIR/issue-84660-unsoundness.rs:12:12
   |
LL | type Bar = impl Foo;
   |            ^^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0119`.