Range pattern does not perform type inference

[nbdd0121]

trait Zero {
    const ZERO: Self;
}

impl Zero for i32 {
    const ZERO: Self = 0;
}

fn foo() {
    match 1 {
        Zero::ZERO ..= 1 => {},
        _ => {},
    }
}

currently throws E0029:

error[E0029]: only `char` and numeric types are allowed in range patterns
  --> src/lib.rs:11:9
   |
11 |         Zero::ZERO ..= 1 => {},
   |         ^^^^^^^^^^     - this is of type `{integer}`
   |         |
   |         this is of type `_` but it should be `char` or numeric

For more information about this error, try `rustc --explain E0029`.

while it should infer the constant to be <i32 as Zero>::ZERO.

@rustbot modify labels: +T-compiler +A-inference
@rustbot claim

[jyn514]

I don't think this is incorrect. Zero::ZERO doesn't refer to a specific constant, it's just an abstract associated constant. i32::ZERO works fine.

[jyn514]

Hmm, let x: i32 = Zero::ZERO does work, though. But let x = Zero::ZERO does not. It does seem inconsistent that let x: i32 = Zero::ZERO works and Zero::ZERO ..= 1_i32 doesn't.

[nbdd0121]

Zero::ZERO is just <_ as Zero>::ZERO, so type inference should kick in here. In fact you can write

match 1 {
    Zero::ZERO => {},
    _ => {},
}

and inference works just fine. The issue only happens when used in range patterns.

I am prototyping a fix right now and it seems that the issue is just that E0029 check happens a little bit early (before unification is performed).