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// compile-flags: -Ztrait-solver=next
// check-pass
// A minimization of an ambiguity when using typenum. See
// https://github.com/rust-lang/trait-system-refactor-initiative/issues/55
// for more details.
trait Id {
type Assoc: ?Sized;
}
impl<T: ?Sized> Id for T {
type Assoc = T;
}
trait WithAssoc<T: ?Sized> {
type Assoc: ?Sized;
}
struct Leaf;
struct Wrapper<U: ?Sized>(U);
impl<U: ?Sized> WithAssoc<U> for Leaf {
type Assoc = U;
}
impl<Ul: ?Sized, Ur: ?Sized> WithAssoc<Wrapper<Ur>> for Wrapper<Ul>
where
Ul: WithAssoc<Ur>,
{
type Assoc = <<Ul as WithAssoc<Ur>>::Assoc as Id>::Assoc;
}
fn bound<T: ?Sized, U: ?Sized, V: ?Sized>()
where
T: WithAssoc<U, Assoc = V>,
{
}
// normalize self type to `Wrapper<Leaf>`
// This succeeds, HOWEVER, instantiating the query response previously
// incremented the universe index counter.
// equate impl headers:
// <Wrapper<Leaf> as WithAssoc<<Wrapper<Leaf> as Id>::Assoc>>
// <Wrapper<?2t> as WithAssoc<Wrapper<?3t>>>
// ~> AliasRelate(<Wrapper<Leaf> as Id>::Assoc, Equate, Wrapper<?3t>)
// add where bounds:
// ~> Leaf: WithAssoc<?3t>
// equate with assoc type:
// ?0t
// <Leaf as WithAssoc<?3t>>::Assoc as Id>::Assoc
// ~> AliasRelate(
// <<Leaf as WithAssoc<?3t>>::Assoc as Id>::Assoc,
// Equate,
// <<Leaf as WithAssoc<?4t>>::Assoc as Id>::Assoc,
// )
//
// We do not reuse `?3t` during generalization because `?0t` cannot name `?4t` as we created
// it after incrementing the universe index while normalizing the self type.
//
// evaluate_added_goals_and_make_query_response:
// AliasRelate(<Wrapper<Leaf> as Id>::Assoc, Equate, Wrapper<?3t>)
// YES, constrains ?3t to Leaf
// AliasRelate(
// <<Leaf as WithAssoc<Leaf>>::Assoc as Id>::Assoc,
// Equate,
// <<Leaf as WithAssoc<?4t>>::Assoc as Id>::Assoc,
// )
//
// Normalizing <<Leaf as WithAssoc<?4t>>::Assoc as Id>::Assoc then *correctly*
// results in ambiguity.
fn main() {
bound::<<Wrapper<Leaf> as Id>::Assoc, <Wrapper<Leaf> as Id>::Assoc, _>()
}
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