Trait inheritance isn't considered in trait bounds checking

[alexcrichton]

trait A: Send {}

fn is_send<T: Send>(){}

fn main() {
    is_send::<~A:>();
}

$ rustc foo.rs
foo.rs:6:5: 6:19 error: instantiating a type parameter with an incompatible type `~A<no-bounds>`, which does not fulfill `Send`                                             
foo.rs:6     is_send::<~A:>();                                                        
             ^~~~~~~~~~~~~~                                                           
error: aborting due to previous error                                                 

I would expect this code to compile because any instance of trait A must already ascribe to Send. This caused some excessive verbosity in #13050.

cc @nikomatsakis
cc @pnkfelix

[nikomatsakis]

This no doubt relates to how type contents handles superbounds and so on. I'd be inclined not to try and fix this until opt-in builtin traits are done.

[reem]

cc me

This is a pretty hairy issue, ergonomics wise.

[nikomatsakis]

This is going to be fixed soon, I suspect, as I am reworking all of this mechanism at the moment as part of opt-in-builtin-kinds and trait reform.

[alexcrichton]

This was fixed quite some time ago.