Repository navigation
Cannot refer to associated type of generics of the same trait (illegal recursive type) #18655
Description
This code:
#![feature(associated_types)]
trait Factory {
type Product;
fn create(&self) -> <Self as Factory>::Product;
}
impl Factory for f64 {
type Product = f64;
fn create(&self) -> f64 { *self * *self }
}
impl<A: Factory, B: Factory> Factory for (A, B) {
type Product = (<A as Factory>::Product, <B as Factory>::Product); // line 14
fn create(&self) -> (<A as Factory>::Product, <B as Factory>::Product) {
let (ref a, ref b) = *self;
(a.create(), b.create())
}
}
fn main() {
println!("{}", (4.0f64, 5.0f64).create());
// ^ should print (16, 25)
}Failed to compile with error:
1.rs:14:20: 14:70 error: illegal recursive type; insert an enum or struct in the cycle, if this is desired
1.rs:14 type Product = (<A as Factory>::Product, <B as Factory>::Product);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I think this should be allowed.
Activity
jroesch commentedon Dec 22, 2014
@nikomatsakis This seems like a bug to me as well. I just encountered it trying to get this code to compile: https://github.com/jroesch/molten/blob/master/src/hlist.rs#L76.
The way I see it the expansion should be as follows.
Given a HList of type:
HCons<int, HCons<int, HNil>>a call totake(S { pred: Z })should result in solving an obligation for an impl ofTake<S<Z>> for HCons<int, HCons<int, HNil>>which incurs a sub obligation to produce an instance ofTake<Z> for HCons<int, HNil>.When checking this we should know
Take<_>::Ralways resolve to anHList(in this case toHNil) and thenHCons<int, <T as Take<Z>>::R>should be equivalent toHCons<int, H0: HList>for the representability check. Soforall T, H: HList. HCons<T, H>can be represented asHCons { head: T, tail: Box<H> }which looks finite to me.Auto merge of #26481 - nham:test-18655, r=arielb1
Rollup merge of rust-lang#81387 - c410-f3r:tests-tests-tests, r=petro…
Rollup merge of rust-lang#81387 - c410-f3r:tests-tests-tests, r=petro…