Confusing mismatched type error message with generic_const_exprs #82619
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Description
I'd be happy to try to fix this, but some pointers on where to start would be greatly appreciated.
Given the following code - playground:
#![allow(incomplete_features)]
#![feature(const_generics)]
#![feature(const_evaluatable_checked)]
use std::ops::Add;
struct ConstWrapper<const N: usize>;
impl<const N1: usize, const N2: usize> Add<ConstWrapper<N2>> for ConstWrapper<N1>
where ConstWrapper<{N1 + N2}>: Sized
{
type Output = ConstWrapper<{N1 + N2}>;
fn add(self, _other: ConstWrapper<N2>) -> Self::Output {
ConstWrapper
}
}
struct WrapConstWrapper<T>(T);
impl<T1: Add<T2>, T2> Add<WrapConstWrapper<T2>> for WrapConstWrapper<T1> {
type Output = WrapConstWrapper<<T1 as Add<T2>>::Output>;
fn add(self, other: WrapConstWrapper<T2>) -> Self::Output {
WrapConstWrapper ( self.0 + other.0 )
}
}
fn get_0() -> WrapConstWrapper<ConstWrapper<0>> {
WrapConstWrapper(ConstWrapper)
}
fn main() {
let x: WrapConstWrapper<ConstWrapper<1>> = get_0() + get_0();
let y = get_0() + get_0();
let z: WrapConstWrapper<ConstWrapper<1>>;
z = y;
}The current output is:
error[E0308]: mismatched types
--> src/main.rs:30:48
|
30 | let x: WrapConstWrapper<ConstWrapper<1>> = get_0() + get_0();
| ^^^^^^^^^^^^^^^^^ expected `1_usize`, found `0_usize`
|
= note: expected type `1_usize`
found type `0_usize`
error[E0308]: mismatched types
--> src/main.rs:33:9
|
33 | z = y;
| ^ expected `1_usize`, found `0_usize`
|
= note: expected struct `WrapConstWrapper<ConstWrapper<1_usize>>`
found struct `WrapConstWrapper<ConstWrapper<0_usize>>`
Ideally, the error messages would be identical. Removing the {N1 + N2} calculation allows the compiler to provide the same error message for both bindings (as seen here). It appears that the compiler is stopping at the mismatched constant instead of evaluating the entire type of get_0() + get_0().
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