Enum variant with GAT field fails to derive Sized

[zygi]

I tried this code:

#![feature(generic_associated_types)]

trait Allocator {
    type Allocated<T>;
}

enum LinkedList<A: Allocator> {
    Head,
    Next(A::Allocated<Self>)
}

fn main() {}

I expected to see this happen: Enum successfully compiles and can be constructed.

Instead, this happened:

error[E0275]: overflow evaluating the requirement '<A as Allocator>::Allocated<CLinkedList<A>>: Sized'

Interestingly, this compiles:

#![feature(generic_associated_types)]

trait Allocator {
    type Allocated<T>;
}

enum CLinkedList<A: Allocator> {
    Head,
    Next(A::Allocated<Self>, i64)
}

fn main() {}

As does this:

#![feature(generic_associated_types)]

trait Allocator {
    type Allocated<T>;
}

struct LinkedListStruct<A: Allocator> {
    value: i64,
    next: Option<A::Allocated<Self>>
}

fn main() {}

Meta

rustc --version --verbose:

rustc 1.51.0-nightly (17eec1433 2021-01-01)
binary: rustc
commit-hash: 17eec1433c69972844dd228b5fe801f218e118c3
commit-date: 2021-01-01
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly

[jedel1043]

Surprisingly, this also doesn't compile:

#![feature(generic_associated_types)]
#![allow(incomplete_features)]

trait Allocator {
    type Allocated<T>;
}

enum LinkedList<A: Allocator> {
    Head,
    Next(i64, A::Allocated<Self>)
}

fn main() {}

error[E0275]: overflow evaluating the requirement '<A as Allocator>::Allocated<LinkedList<A>>: Sized'

[4lDO2]

I have checked that changing type Allocated<T>; to type Allocated<T>; Sized has no effect at all, as to be expected. But when opting out of Sized using type Allocated<T>: ?Sized; seems to prevent it from overflowing, though still obviously not compiling.

[aleksmelnikov]

Examples that compile:

#![feature(generic_associated_types)]
#![allow(incomplete_features)]

trait Allocator {
    type Allocated<T>;
}

enum LinkedList<A: Allocator> {
    Head,
    Next(Box<A::Allocated<Self>>)
}

fn main() {}

#![feature(generic_associated_types)]
#![allow(incomplete_features)]

trait Allocator {
    type Allocated<T>;
}

enum LinkedList<'a, A: Allocator> {
    Head,
    Next(&'a A::Allocated<Self>)
}

fn main() {}

#![feature(generic_associated_types)]
#![allow(incomplete_features)]

trait Allocator {
    type Allocated<T>;
}

enum LinkedList<A: Allocator> where Self: Sized {
    Head,
    Next(A::Allocated<Self>)
}

fn main() {}

[jackh726]

GATs issue triage: not blocking. While this bug is unfortunate, this isn't really a GATs problem, per se. Proper fix is probably a coinductive Sized. Workaround is a ?Sized bound.

[Dylan-DPC]

This code now compiles fine without the above error