error: cannot implement trait on type alias impl trait
  --> $DIR/issue-84660-trait-impl-for-tait.rs:15:15
   |
LL | impl TraitArg<Bar> for () {
   |               ^^^
   |
note: type alias impl trait defined here
  --> $DIR/issue-84660-trait-impl-for-tait.rs:8:12
   |
LL | type Bar = impl Foo;
   |            ^^^^^^^^

error: aborting due to previous error