Duplicate `E0121` errors are emitted for placeholder types `_` in some positions

[varkor]

type FOO = fn()  -> _;

fn main() {}

produces (playground link):

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> src/main.rs:1:21
  |
1 | type FOO = fn()  -> _;
  |                     ^
  |                     |
  |                     not allowed in type signatures
  |                     help: use type parameters instead: `T`

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> src/main.rs:1:21
  |
1 | type FOO = fn()  -> _;
  |                     ^ not allowed in type signatures

error: aborting due to 2 previous errors

[reza-ebrahimi]

Just happening in Beta and Nightly channel.

[estebank]

Triage: no change.