error[E0792]: expected generic type parameter, found `i32`
  --> $DIR/issue-77179.rs:8:14
   |
LL | type Pointer<T> = impl std::ops::Deref<Target = T>;
   |              - this generic parameter must be used with a generic type parameter
...
LL | fn test() -> Pointer<_> {
   |              ^^^^^^^^^^

error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
  --> $DIR/issue-77179.rs:8:22
   |
LL | fn test() -> Pointer<_> {
   |                      ^ not allowed in type signatures
   |
help: replace with the correct return type
   |
LL - fn test() -> Pointer<_> {
LL + fn test() -> Pointer<i32> {
   |

error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
  --> $DIR/issue-77179.rs:19:25
   |
LL |     fn bar() -> Pointer<_>;
   |                         ^ not allowed in type signatures

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0121, E0792.
For more information about an error, try `rustc --explain E0121`.