error[E0277]: the trait bound `(): Add<A>` is not satisfied
  --> $DIR/associated-types-ICE-when-projecting-out-of-err.rs:23:11
   |
LL |     r = r + a;
   |           ^ the trait `Add<A>` is not implemented for `()`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.