rustdoc: synthetic impls are faulty in the presence of assoc const equality constraints

[fmease]

Given

#![feature(associated_const_equality)]

use std::marker::Unpin; // or any other auto trait

pub struct Outer<T>(Inner<T>);
pub struct Inner<T>(T);

// (A)
impl<const F: bool, T: Trait<FLAG = {F}>> Unpin for Inner<T> {}

// (B)
//impl<T: Trait<FLAG = true>> Unpin for Inner<T> {}

pub trait Trait {
    const FLAG: bool;
}

the impl of Unpin (or any other auto trait chosen) synthesized for the generic type Outer looks like

impl<T> !Unpin for Outer<T> // for either case, (A) or (B)
//      ^ note the `!`

while I expected the impls to be

// for case (A):
impl<T> Unpin for Outer<T> where T: Trait 
// for case (B):
impl<T> Unpin for Outer<T> where T: Trait<FLAG = true>

For comparison, the analog involving type equality constraints

use std::marker::Unpin; // or any other auto trait

pub struct Outer<T>(Inner<T>);
pub struct Inner<T>(T);

// (A)
impl<F: Bool, T: Trait<Flag = F>> Unpin for Inner<T> {}

//   ---> synthetic impl:
//   impl<T> Unpin for Outer<T> where T: Trait

// (B)
// impl<T: Trait<Flag = True>> Unpin for Inner<T> {}

//   ---> synthetic impl:
//   impl<T> Unpin for Outer<T> where T: Trait<Flag = True>

pub trait Trait {
    type Flag: Bool;
}

pub trait Bool {}
impl Bool for True {}
pub enum True {}

(adding T-compiler next to T-rustdoc since AutoTraitFinder lives in rustc)

@rustbot label C-bug T-rustdoc T-compiler A-auto-traits A-synthetic-impls F-associated_const_equality