Additional trait restrictions prevent use of trait methods #44064
Description
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)