Extra "type annotations needed" error after trying to constrain a non-existent type parameter with the same trait bound as another type parameter

[jruderman]

Given the following code:

use core::ops::Add;

fn dbl<T>(x: T) -> <T as Add>::Output
where
    T: Copy + Add,
    UUU: Copy
{
    x + x
}

fn main() {
    println!("{}", dbl(3));
}

The current output is:

error[[E0412]](https://doc.rust-lang.org/nightly/error-index.html#E0412): cannot find type `UUU` in this scope
 --> src/main.rs:6:5
  |
6 |     UUU: Copy
  |     ^^^ not found in this scope

error[[E0282]](https://doc.rust-lang.org/nightly/error-index.html#E0282): type annotations needed
 --> src/main.rs:3:8
  |
3 | fn dbl<T>(x: T) -> <T as Add>::Output
  |        ^ cannot infer type for type parameter `T`

Similarly, this code:

use core::ops::Add;

fn dbl<T>(x: T) -> <T as Add>::Output
where
    T: Copy + Add,
    UUU: Add
{
    x + x
}

fn main() {
    println!("{}", dbl(3));
}

produces

error[[E0412]](https://doc.rust-lang.org/nightly/error-index.html#E0412): cannot find type `UUU` in this scope
 --> src/main.rs:6:5
  |
6 |     UUU: Add
  |     ^^^ not found in this scope

error[[E0284]](https://doc.rust-lang.org/nightly/error-index.html#E0284): type annotations needed
 --> src/main.rs:3:1
  |
3 | / fn dbl<T>(x: T) -> <T as Add>::Output
4 | | where
5 | |     T: Copy + Add,
6 | |     UUU: Add
7 | | {
8 | |     x + x
9 | | }
  | |_^ cannot infer type
  |
  = note: cannot satisfy `<T as Add>::Output == _`

In both cases, the "type annotation needed" error (E0282 or E0284) shouldn't be shown.

Ideally, the erroneous UUU line should not prevent type inference from succeeding, because UUU does not refer to anything. Alternatively (as in #88630, #75331, #68507) the first error could cancel the attempt at inferring types.