invalid syntax in "consider further restricting this bound" when there is already a bound present

[lolbinarycat]

Code

trait Pair {
    type Left;
    type Right;
    
    fn cons(left: Self::Left, right: Self::Right) -> Self;
    fn split(self) -> (Self::Left, Self::Right);
}

impl<A, B> Pair for (A, B) {
    type Left = A;
    type Right = B;
    
    fn cons(left: Self::Left, right: Self::Right) -> Self {
        (left, right)
    }
    
    fn split(self) -> (Self::Left, Self::Right) {
        self
    }
}

fn frob<A, B>(pair: impl Pair<Left = A>) -> impl Pair<Left = A, Right = B> {
    let (left, right) = pair.split();
    (left, right)
}

Current output

error[E0271]: type mismatch resolving `<(A, <impl Pair<Left = A> as Pair>::Right) as Pair>::Right == B`
  --> src/lib.rs:22:45
   |
22 | fn frob<A, B>(pair: impl Pair<Left = A>) -> impl Pair<Left = A, Right = B> {
   |            - expected this type parameter   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type mismatch resolving `<(A, <impl Pair<Left = A> as Pair>::Right) as Pair>::Right == B`
23 |     let (left, right) = pair.split();
24 |     (left, right)
   |     ------------- return type was inferred to be `(A, <impl Pair<Left = A> as Pair>::Right)` here
   |
note: expected this to be `B`
  --> src/lib.rs:11:18
   |
11 |     type Right = B;
   |                  ^
   = note: expected type parameter `B`
             found associated type `<impl Pair<Left = A> as Pair>::Right`
help: consider further restricting this bound
   |
22 | fn frob<A, B>(pair: impl Pair<Left = A><Right = B>) -> impl Pair<Left = A, Right = B> {
   |                                        +++++++++++

For more information about this error, try `rustc --explain E0271`.

Desired output

error[E0271]: type mismatch resolving `<(A, <impl Pair<Left = A> as Pair>::Right) as Pair>::Right == B`
  --> src/lib.rs:22:45
   |
22 | fn frob<A, B>(pair: impl Pair<Left = A>) -> impl Pair<Left = A, Right = B> {
   |            - expected this type parameter   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type mismatch resolving `<(A, <impl Pair<Left = A> as Pair>::Right) as Pair>::Right == B`
23 |     let (left, right) = pair.split();
24 |     (left, right)
   |     ------------- return type was inferred to be `(A, <impl Pair<Left = A> as Pair>::Right)` here
   |
note: expected this to be `B`
  --> src/lib.rs:11:18
   |
11 |     type Right = B;
   |                  ^
   = note: expected type parameter `B`
             found associated type `<impl Pair<Left = A> as Pair>::Right`
help: consider further restricting this bound
   |
22 | fn frob<A, B>(pair: impl Pair<Left = A, Right = B>) -> impl Pair<Left = A, Right = B> {
   |                                         +++++++++++

For more information about this error, try `rustc --explain E0271`.

Rationale and extra context

No response

Other cases

Rust Version

rustc 1.89.0-nightly (1677d46cb 2025-06-10)
binary: rustc
commit-hash: 1677d46cb128cc8f285dbd32b0dc4d7a46437050
commit-date: 2025-06-10
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

Anything else?

No response