Malformed suggestion for filling in multiple placeholder return lifetimes

[wabain]

Playground.

I tried this code:

struct TwoLifetimes<'x, 'y> {
    x: &'x (),
    y: &'y (),
}

fn two_lifetimes_needed(a: &(), b: &()) -> TwoLifetimes<'_, '_> {
    TwoLifetimes { x: &(), y: &() }
}

I expected to see an error message suggesting adding a named lifetime 'a to the returned type TwoLifetimes<'a, 'a>.

Instead, the suggested substitution is the malformed TwoLifetimes<'_<'a, 'a>, '_>.

error[E0106]: missing lifetime specifiers
  --> src/main.rs:11:57
   |
11 | fn two_lifetimes_needed(a: &(), b: &()) -> TwoLifetimes<'_, '_> {
   |                            ---     ---                  ^^ expected 2 lifetime parameters
   |
   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `a` or `b`
help: consider introducing a named lifetime parameter
   |
11 | fn two_lifetimes_needed<'a>(a: &'a (), b: &'a ()) -> TwoLifetimes<'_<'a, 'a>, '_> {
   |                        ^^^^    ^^^^^^     ^^^^^^                  ^^^^^^^^^^

The linked playground shows that the output is correct when the return type lifetimes are elided (a bare TwoLifetimes), and also if only one placeholder lifetime is used (OneLifetime<'_>).

Meta

Reproduced on 1.53.0-nightly (2021-04-25 3709ae3), on 1.51.0 stable, and 1.47.0 stable.