45 lines
1.6 KiB
Rust
45 lines
1.6 KiB
Rust
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// This must fail coherence.
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//
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// Getting this to pass was fairly difficult, so here's an explanation
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// of what's happening:
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//
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// Normalizing projections currently tries to replace them with inference variables
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// while emitting a nested `Projection` obligation. This cannot be done if the projection
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// has bound variables which is the case here.
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//
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// So the projections stay until after normalization. When unifying two projections we
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// currently treat them as if they are injective, so we **incorrectly** unify their
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// substs. This means that coherence for the two impls ends up unifying `?T` and `?U`
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// as it tries to unify `<?T as WithAssoc1<'a>>::Assoc` with `<?U as WithAssoc1<'a>>::Assoc`.
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//
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// `impl1` therefore has the projection `<?T as WithAssoc2<'a>>::Assoc` and we have the
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// assumption `?T: for<'a> WithAssoc2<'a, Assoc = i32>` in the `param_env`, so we normalize
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// that to `i32`. We then try to unify `i32` from `impl1` with `u32` from `impl2` which fails,
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// causing coherence to consider these two impls distinct.
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// compile-flags: -Ztrait-solver=next
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pub trait Trait<T> {}
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pub trait WithAssoc1<'a> {
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type Assoc;
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}
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pub trait WithAssoc2<'a> {
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type Assoc;
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}
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// impl 1
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impl<T, U> Trait<for<'a> fn(<T as WithAssoc1<'a>>::Assoc, <U as WithAssoc2<'a>>::Assoc)> for (T, U)
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where
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T: for<'a> WithAssoc1<'a> + for<'a> WithAssoc2<'a, Assoc = i32>,
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U: for<'a> WithAssoc2<'a>,
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{
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}
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// impl 2
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impl<T, U> Trait<for<'a> fn(<U as WithAssoc1<'a>>::Assoc, u32)> for (T, U) where
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U: for<'a> WithAssoc1<'a> //~^ ERROR conflicting implementations of trait
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{
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}
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fn main() {}
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