Additional trait restrictions prevent use of trait methods

[Xaeroxe]

Adding Send + Sync restrictions to an Any trait object causes the trait object to lose the downcast_ref method

https://play.rust-lang.org/?gist=9c665d721a9c6a2845f55e56d97ffe27&version=stable

use std::any::Any;

struct Foo;

// This compiles
fn main() {
    let foo: Box<Any> = Box::new(Foo) as Box<Any>;
    let foo = &foo;
    let foo = foo.downcast_ref::<Foo>().unwrap();
}

// This compiles
fn main_2() {
    let foo: Box<Any + Send + Sync> = Box::new(Foo) as Box<Any + Send + Sync>;
    let foo = &foo;
    let foo = Any::downcast_ref::<Foo>(foo).unwrap();
}

// This doesn't compile
fn main_3() {
    let foo: Box<Any + Send + Sync> = Box::new(Foo) as Box<Any + Send + Sync>;
    let foo = &foo;
    let foo = foo.downcast_ref::<Foo>().unwrap();
}

I would expect main_3 to compile, because main_2 compiles, however main_3 fails to compile with the error

error[E0599]: no method named `downcast_ref` found for type `&std::boxed::Box<std::any::Any + std::marker::Send + std::marker::Sync>` in the current scope
  --> src/main.rs:23:19
   |
23 |     let foo = foo.downcast_ref::<Foo>().unwrap();
   |                   ^^^^^^^^^^^^

error: aborting due to previous error(s)

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

Meta

$ rustc --version
rustc 1.19.0 (0ade339 2017-07-17)

[kennytm]

Any + Send also works, since there is an impl for it. This needs a separate impl due to #23423, in order to fix #18737.

So to fix this we just need to forward Any + Sync and Any + Send + Sync also.

[Xaeroxe]

After thoughts on filing this issue:

I guess the question is "should adding a provided method on a trait be a breaking change for multi-trait trait objects in the event that ambiguity between methods for multiple traits is introduced by the provided method's presence?" Which I think the answer is no, so yeah it makes sense to prevent method calls in this way, but perhaps the compiler could give a better syntax hint as to what should be done instead.

I wouldn't be opposed to an impl for Any + Send + Sync but I also don't think it's terribly necessary.

[Xaeroxe]

I also don't think it's terribly necessary.

So I'm going to close this issue.