using shadowed impls may add unnecessary region constraints

[lcnr]

trait Trait<'a> {
    type Assoc;
}

impl<T> Trait<'static> for T {
    type Assoc = ();
}

// normalizing requires `'a == 'static`, the trait bound does not.
fn foo<'a, T: Trait<'a>>(_: T::Assoc) {}

Originally posted by @lcnr in #12 (comment)

[lcnr]

in the following issue both the where-bound and the impl add different region requirements

trait Trait<'a> {
    type Assoc;
}
impl Trait<'static> for u32 {
    type Assoc = u32;
}

fn bar<'a, T: Trait<'a>>() -> T::Assoc { todo!() }
fn foo<'a>()
where
    u32: Trait<'a>,
{
    bar::<u32>();
}

cc #129 (comment)