FP `non-exhaustive pattern: `_` not covered`  recursive RPIT

```rust
fn rpit<'a: 'b, 'b: 'a>() -> impl Sized {
    let () = rpit::<'b, 'a>();
}
fn main() {}
```
this compiles with rustc but RA diagnostics show an error here
```
at crate ra, file /tmp/ra/src/main.rs: Error RustcHardError("E0005") from LineCol { line: 1, col: 8 } to LineCol { line: 1, col: 10 }: non-exhaustive pattern: `_` not covered
```