Rust: Inherited trait bounds are not completely checked

[behnam]

Original discussion: https://users.rust-lang.org/t/unexpected-behaviors-of-trait-bounds/12286/10

Looks like it's related to this old issue: https://github.com/rust-lang/rust/issues/29859

Basically, having trait Complete require trait Partial, and trait Partial require trait PartialEq (or any other trait), a type (struct TypeB) with impl Complete doesn't need to satisfy impl PartialEq, which only looks like a bug.

Interestingly, whenever <TypeB as Partial> is seen, the check is performed and appropriate error is thrown.

Here's a repro:

pub trait Partial: PartialEq {
    fn foo() -> Option<bool>;
}

pub trait Complete: Partial {
    fn foo() -> bool;
}

impl<T> Partial for T
where
    T: Complete,
{
    fn foo() -> Option<bool> {
        Some(<Self as Complete>::foo())
    }
}


// ----

#[derive(PartialEq)]
pub struct TypeA {}

impl Partial for TypeA {
    fn foo() -> Option<bool> {
        None
    }
}


// ----

// BUG: No compile warning about `PartialEq` not being implemented
// #[derive(PartialEq)]
pub struct TypeB {}

impl Complete for TypeB {
    fn foo() -> bool {
        true
    }
}


// ----

pub fn main() {
    println!("{:?}", TypeA::foo());

    // This works, but shouldn't!
    println!("{:?}", <TypeB as Complete>::foo());

    // This would trigger the issue, though.
    //println!("{:?}", <TypeB as Partial>::foo());
    /* Result in:
    error[E0277]: the trait bound `TypeB: std::cmp::PartialEq` is not satisfied
      --> src/bin/cyclic_traits.rs:48:22
       |
    48 |     println!("{:?}", <TypeB as Partial>::foo());
       |                      ^^^^^^^^^^^^^^^^^^^^^^^ can't compare `TypeB` with `TypeB`
       |
       = help: the trait `std::cmp::PartialEq` is not implemented for `TypeB`
       = note: required by `Partial::foo`
    */
}

[dtolnay]

Here is a minimized repro in ICE form:

trait Base {
    fn base() {}
}

trait Partial: Base {}
trait Complete: Partial {}

impl<T: Complete> Partial for T {}

struct TypeB;
impl Complete for TypeB {}

fn main() {
    ice::<TypeB>();
}

fn ice<P: Partial>() {
    P::base();
}

error: internal compiler error: /checkout/src/librustc/traits/trans/mod.rs:75: Encountered error `Unimplemented` selecting `Binder(<TypeB as Base>)` during trans

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.21.0-nightly (cbbe17aa7 2017-08-07) running on x86_64-unknown-linux-gnu

[scalexm]

Seems like @arielb1 also opened his own issue (#43784) for this. Anyway, this is fixed (well should be) by #43786.

[nikomatsakis]

I believe this is a dup of #43784, closing.