blob: 794d677c8b657679f35f842fd1a38c5928e0128a (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
|
// Like `projection-bound-cycle.rs` but this avoids using
// `feature(trivial_bounds)`.
#![feature(generic_associated_types)]
trait Print {
fn print();
}
trait Foo {
type Item: Sized where <Self as Foo>::Item: Sized;
}
struct Number<T> { t: T }
impl<T> Foo for Number<T> {
// Well-formedness checks require that the following
// goal is true:
// ```
// if ([T]: Sized) { # if the where clauses hold
// [T]: Sized # then the bound on the associated type hold
// }
// ```
// which it is :)
type Item = [T] where [T]: Sized;
}
struct OnlySized<T> where T: Sized { f: T }
impl<T> Print for OnlySized<T> {
fn print() {
println!("{}", std::mem::size_of::<T>());
}
}
trait Bar {
type Assoc: Print;
}
impl<T> Bar for T where T: Foo {
// This is not ok, we need to prove `wf(<T as Foo>::Item)`, which requires
// knowing that `<T as Foo>::Item: Sized` to satisfy the where clause. We
// can use the bound on `Foo::Item` for this, but that requires
// `wf(<T as Foo>::Item)`, which is an invalid cycle.
type Assoc = OnlySized<<T as Foo>::Item>;
//~^ ERROR overflow evaluating the requirement `<T as Foo>::Item: Sized`
}
fn foo<T: Print>() {
T::print() // oops, in fact `T = OnlySized<str>` which is ill-formed
}
fn bar<T: Bar>() {
// we have `FromEnv(T: Bar)` hence
// `<T as Bar>::Assoc` is well-formed and
// `Implemented(<T as Bar>::Assoc: Print)` hold
foo::<<T as Bar>::Assoc>()
}
fn main() {
bar::<Number<u8>>()
}
|
