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// build-pass | ||
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#![feature(type_alias_impl_trait)] | ||
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use std::marker::PhantomData; | ||
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/* copied Index and TryFrom for convinience (and simplicity) */ | ||
trait MyIndex<T> { | ||
type O; | ||
fn my_index(self) -> Self::O; | ||
} | ||
trait MyFrom<T>: Sized { | ||
type Error; | ||
fn my_from(value: T) -> Result<Self, Self::Error>; | ||
} | ||
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/* MCVE starts here */ | ||
trait F {} | ||
impl F for () {} | ||
type DummyT<T> = impl F; | ||
fn _dummy_t<T>() -> DummyT<T> {} | ||
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struct Phantom1<T>(PhantomData<T>); | ||
struct Phantom2<T>(PhantomData<T>); | ||
struct Scope<T>(Phantom2<DummyT<T>>); | ||
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impl<T> Scope<T> { | ||
fn new() -> Self { | ||
unimplemented!() | ||
} | ||
} | ||
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impl<T> MyFrom<Phantom2<T>> for Phantom1<T> { | ||
type Error = (); | ||
fn my_from(_: Phantom2<T>) -> Result<Self, Self::Error> { | ||
unimplemented!() | ||
} | ||
} | ||
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impl<T: MyFrom<Phantom2<DummyT<U>>>, U> MyIndex<Phantom1<T>> for Scope<U> { | ||
type O = T; | ||
fn my_index(self) -> Self::O { | ||
MyFrom::my_from(self.0).ok().unwrap() | ||
} | ||
} | ||
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fn main() { | ||
let _pos: Phantom1<DummyT<()>> = Scope::new().my_index(); | ||
} |