TAIT shouldn't constrain impl generics

[aliemjay]

The following UB compiles: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=4cc89e9d4c133a6398f297e6f56e07f2

#![feature(type_alias_impl_trait)]
use core::fmt::Display;

type Opaque<'a> = impl Sized + 'static;
fn define<'a>() -> Opaque<'a> {}

trait Trait {
    type Assoc: Display;
}
impl<'a> Trait for Opaque<'a> {
    type Assoc = &'a str;
}

// ======= Exploit =======

fn extend<T: Trait + 'static>(s: T::Assoc) -> Box<dyn Display> {
    Box::new(s)
}

fn main() {
    let val = extend::<Opaque<'_>>(&String::from("blah blah blah"));
    println!("{}", val);
}

The impl is invalid because the lifetime parameter 'a appears unconstrained in the impl header. It should be rejected for the same reason that projection types doesn't constraint impl generics:

impl<'a> Trait for <Opaque<'a> as Trait>::Assoc {
//~^ ERROR the lifetime parameter `'a` is not constrained
    type Assoc = &'a str;
}

@rustbot label C-bug T-types I-unsound requires-nightly F-type_alias_impl_trait