| // Like `projection-bound-cycle.rs` but this avoids using |
| // `feature(trivial_bounds)`. |
| |
| #![feature(generic_associated_types)] |
| //~^ WARNING the feature `generic_associated_types` is incomplete |
| |
| 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 where [T]: Sized = [T]; |
| } |
| |
| 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>>() |
| } |