E0121 hint has duplicated type param with lifetime

[l4l]

The following code:

struct X<'a, T>(&'a T);

impl X<'_, _> {}

leads to the following error (tested at playground with 1.48 stable & 1.50 nightly):

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> src/lib.rs:3:12
  |
3 | impl X<'_, _> {}
  |            ^ not allowed in type signatures
  |
help: use type parameters instead
  |
3 | impl X<'_, T, T> {}
  |          ^^^  ^

error: aborting due to previous error