error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
  --> $DIR/issue-98260.rs:3:27
   |
LL |     fn a(aa: B) -> Result<_, B> {
   |                           ^ not allowed in type signatures
   |
help: replace with the correct return type
   |
LL -     fn a(aa: B) -> Result<_, B> {
LL +     fn a(aa: B) -> Result<(), B> {
   |

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0121`.