candidate selection for normalization and trait goals disagree

[lcnr]

#![feature(discriminant_kind)]
use std::marker::DiscriminantKind;

fn trait_bound<T: DiscriminantKind>() {}
fn normalize<T: DiscriminantKind<Discriminant = u8>>() {}

fn foo<'a, 'b>()
where
    &'b (): DiscriminantKind<Discriminant = u8>,    
{
    trait_bound::<&'a ()>();
}

fn bar<'a, 'b>()
where
    &'b (): DiscriminantKind<Discriminant = u8>,    
{
    normalize::<&'a ()>();
}

foo compiles, bar does not:

error: lifetime may not live long enough
  --> src/lib.rs:18:5
   |
14 | fn bar<'a, 'b>()
   |        --  -- lifetime `'b` defined here
   |        |
   |        lifetime `'a` defined here
...
18 |     normalize::<&'a ()>();
   |     ^^^^^^^^^^^^^^^^^^^ requires that `'b` must outlive `'a`
   |
   = help: consider adding the following bound: `'b: 'a`

error: lifetime may not live long enough
  --> src/lib.rs:18:5
   |
14 | fn bar<'a, 'b>()
   |        --  -- lifetime `'b` defined here
   |        |
   |        lifetime `'a` defined here
...
18 |     normalize::<&'a ()>();
   |     ^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'b`
   |
   = help: consider adding the following bound: `'a: 'b`

help: `'b` and `'a` must be the same: replace one with the other

Candidate selection for the trait goal prefers the trivial builtin impl. Normalization instead prefers the where-bound. This is inconsistent and means that whether we use the associated items impacts whether a trait bound holds.

It impacts all trivial builtin traits with associated types, I don't think this effects stable rn as either the trait is unstable or the builtin impls only exist for unnameable types. Nominating for t-types vibeck

[lcnr]

I realize this also affects our preference of impls over global where-bounds:

trait Trait {
    type Assoc;
}

impl Trait for &u32 {
    type Assoc = u32;
}

fn trait_bound<T: Trait>() {}
fn normalize<T: Trait<Assoc = u32>>() {}

fn foo<'a>()
where
    &'static u32: Trait<Assoc = u32>,
{
    trait_bound::<&'a u32>(); // ok
    normalize::<&'a u32>(); // error
}

[lcnr]

we also have the same issue with alias-bound/where-bound mismatch

trait Bound<'a> {
    type Assoc;
}
trait Trait {
    type Assoc: Bound<'static, Assoc = u32>;
}

fn heck<'a, T: Trait<Assoc: Bound<'a>>>(x: <T::Assoc as Bound<'a>>::Assoc) {
    drop(x);
}

[lcnr]

we also have the same issue with alias-bound/where-bound mismatch

trait Bound<'a> {
    type Assoc;
}
trait Trait {
    type Assoc: Bound<'static, Assoc = u32>;
}

fn heck<'a, T: Trait<Assoc: Bound<'a>>>(x: <T::Assoc as Bound<'a>>::Assoc) {
    drop(x);
}