Rust's coherence checking overconservative

[minhnhdo]

Code:

use std::rand;

pub trait Nothing { }

impl<T: rand::Rand> Nothing for T { }
impl<T: rand::Rand> Nothing for ~[T] { }

Output:

error: conflicting implementations for trait `lib::Nothing`

Expected:
Compile normally, as there is no impl Rand for ~[T].

[thestinger]

This is the intended behaviour. A crate can only define a trait if either the type it's defining it for or the trait is local to the crate. This prevents libraries from being able to create conflicting trait implementations. It wouldn't be good if Rust libraries could be incompatible with linking to each other from a basic language feature like this.

[huonw]

@thestinger I believe that rule means that this can be valid, because the compiler knows that someone can't define Rand for ~[] (because any (trait, type) pair visible in this crate has whether it is implemented or not also visible).

[thestinger]

Ah, sorry, I missed that the Nothing trait was local.

[nikomatsakis]

cc me. cc #5527.

[japaric]

This is now working because of conditional/multi dispatch.

@nikomatsakis can we close this?

[nikomatsakis]

Yes.