warning: elided lifetime has a name
  --> $DIR/impl-fn-predefined-lifetimes.rs:4:48
   |
LL | fn a<'a>() -> impl Fn(&'a u8) -> (impl Debug + '_) {
   |      -- lifetime `'a` declared here            ^^ this elided lifetime gets resolved as `'a`
   |
   = note: `#[warn(elided_named_lifetimes)]` on by default

error[E0792]: expected generic lifetime parameter, found `'_`
  --> $DIR/impl-fn-predefined-lifetimes.rs:7:9
   |
LL | fn a<'a>() -> impl Fn(&'a u8) -> (impl Debug + '_) {
   |                                                -- this generic parameter must be used with a generic lifetime parameter
...
LL |     |x| x
   |         ^

error[E0720]: cannot resolve opaque type
  --> $DIR/impl-fn-predefined-lifetimes.rs:4:35
   |
LL | fn a<'a>() -> impl Fn(&'a u8) -> (impl Debug + '_) {
   |                                   ^^^^^^^^^^^^^^^ recursive opaque type
...
LL |     |x| x
   |     ----- returning here with type `{closure@$DIR/impl-fn-predefined-lifetimes.rs:7:5: 7:8}`

error: aborting due to 2 previous errors; 1 warning emitted

Some errors have detailed explanations: E0720, E0792.
For more information about an error, try `rustc --explain E0720`.