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//@ compile-flags: -Znext-solver
//@ check-pass
// A regression test for trait-system-refactor-initiative#161
trait Constrain<T> {
type Assoc;
}
impl<T> Constrain<T> for () {
type Assoc = ();
}
struct Foo<T, U = <() as Constrain<T>>::Assoc>(T, U);
impl<T: Copy> Foo<T> {
fn foo() {}
}
struct B;
impl Foo<B> {
fn foo() {}
}
type Alias<T> = Foo<T>;
fn via_guidance<T: Copy>()
where
(): Constrain<T>,
{
// Method selection on `Foo<?t, <() as Constrain<?t>>::Assoc>` is ambiguous.
// only by unnecessarily constraining `?t` to `T` when proving `(): Constrain<?t>`
// are we able to select the first impl.
//
// This happens in the old solver when normalizing `Alias<?t>`. The new solver doesn't try
// to eagerly normalize `<() as Constrain<?t>>::Assoc` so we instead always prove that the
// self type is well-formed before method lookup.
Alias::foo();
}
fn main() {
via_guidance::<()>();
}
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