Misleading "`match` arms have incompatible types" when match expression used in unimplemented operation

[LilyIsTrans]

Code

pub struct Instant(chrono::NaiveDateTime);

pub fn example() {
    use std::ops::Bound;
    use std::ops::RangeBounds;

    let hours = Instant(chrono::NaiveDateTime::MIN)..=Instant(chrono::NaiveDateTime::MAX);

    let total_hours = match hours.end_bound() {
        Bound::Included(a) => a,
        _ => unreachable!(),
    } - match hours.start_bound() {
        Bound::Included(a) => a,
        _ => unreachable!(),
    };
}

Current output

error[E0308]: `match` arms have incompatible types
  --> src/lib.rs:22:14
   |
20 |       } - match hours.start_bound() {
   |  _________-
21 | |         Bound::Included(a) => a,
   | |                               - this is found to be of type `&Instant`
22 | |         _ => unreachable!(),
   | |              ^^^^^^^^^^^^^^ expected `&Instant`, found `!`
23 | |     };
   | |_____- `match` arms have incompatible types
   |
   = note: expected reference `&Instant`
                   found type `!`
   = note: this error originates in the macro `$crate::panic::unreachable_2021` which comes from the expansion of the macro `unreachable` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0308`.

Desired output

error[E0369]: cannot subtract `&Instant` from `&Instant`
   --> src/lib.rs:20:7
    |
20  |    } - match hours.start_bound() {
    |      ^ no implementation for `&Instant - &Instant`
    |
note: an implementation of `Sub` might be missing for `Instant`
   --> src/lib.rs:9:1
    |
9   | pub struct Instant(chrono::NaiveDateTime);
    | ^^^^^^^^^^^^^^^^^^ must implement `Sub`
note: the trait `Sub` must be implemented
   --> /home/lily/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/arith.rs:184:1
    |
184 | pub trait Sub<Rhs = Self> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0369`.

Rationale and extra context

The error goes away if std::ops::Sub is implemented for &Instant

Other cases

Output is identical on latest stable, beta, and nightly toolchains

Anything else?

I think this might be related to this issue (actually I initially thought it was the same, but that issue appears to have been fixed in the latest beta and nightly releases, while this has not).

[jieyouxu]

@rustbot label +D-confusing +A-typesystem +T-types -needs-triage