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