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