Projecting <Type as Trait>::N should check that N is part of Trait

[pnkfelix]

spawned off of #21259

test case

use std::default::Default;

trait Impled { fn f(&self) {} }
trait Vr: Impled {
    type Y: Default + Par<<Self as Impled>::Noexist>; fn g(&self) {}
}
trait Par<X:Default> {
    fn h(&self) -> (X, String) { (Default::default(), format!("Par::h")) }
}

struct S1;
impl Impled for S1 {}
impl Vr for S1 { type Y = (); }
impl Par<()> for () { }

fn main() {
    let y: <S1 as Vr>::Y = Default::default();
    let (_, s) = Par::h(&y);
    println!("{}", s);
}

The above code should be rejected by the compiler, because <Self as Impled>::Noexist is not a well-formed projection of an associated item. Instead, it currently runs.

---

(This bug is not quite as bad as it might seem, since one easily runs into problems if one actually attempts to program generically with this trait and associated item, as shown in the below modification to the last three lines above. This new program is correctly rejected by the compiler, though perhaps with messages that are ... subpar:)

fn foo<V:Vr>(v: V) {
    let y: <V as Vr>::Y = Default::default();
    let (_, s) = Par::h(&y);
    println!("{}", s);
}
fn main() {
    foo(S1);
}

Transcript of error output:

../src/test/compile-fail/issue-21259-e.rs:18:18: 18:24 error: the trait `core::marker::Sized` is not implemented for the type `<V as Impled>::Noexist` [E0277]
../src/test/compile-fail/issue-21259-e.rs:18     let (_, s) = Par::h(&y);
                                                              ^~~~~~
../src/test/compile-fail/issue-21259-e.rs:18:18: 18:24 note: `<V as Impled>::Noexist` does not have a constant size known at compile-time
../src/test/compile-fail/issue-21259-e.rs:18     let (_, s) = Par::h(&y);
                                                              ^~~~~~
../src/test/compile-fail/issue-21259-e.rs:18:18: 18:24 error: the trait `core::default::Default` is not implemented for the type `<V as Impled>::Noexist` [E0277]
../src/test/compile-fail/issue-21259-e.rs:18     let (_, s) = Par::h(&y);
                                                              ^~~~~~
error: aborting due to 2 previous errors

[pnkfelix]

see also #19883 , which fixed a very similar bug, but presumably did so in a much more roundabout manner than what I am attempting.

---

update: Oh I see, someone thought the issue was fixed because they started seeing failures on the particular test case that had been filed, and so they just added tests codifying those (strange) error messages.

[pnkfelix]

This should be fixed by #22172. But I want to port the regression tests over from #22759.

[bluss]

I think this is fixed, I see

<anon>:5:27: 5:52 error: use of undeclared associated type `Impled::Noexist`
<anon>:5     type Y: Default + Par<<Self as Impled>::Noexist>; fn g(&self) {}
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~

👍

[pnkfelix]

And at this point I do not know what regression tests my earlier comment was referencing; there are not any such tests on #22759 ...

[pnkfelix]

Hmm, maybe I meant to reference #21259 and just mistyped? Or maybe I meant to refer to this issue itself? Reopened so that I'll attempt to put in the corresponding tests.

[arielb1]

This is fixed & tested.