There should be a Box<Error+Send+Sync> -> Box<Error> From conversion

[talchas]

This conversion is done implicitly, but implicit conversions aren't done in generic lookup. This means that

fn foo() -> Result<(), Box<Error + Send + Sync>> { try!(Err("foo")) }
fn bar() -> Result<(), Box<Error>> { try!(foo()); }

will fail, which is obnoxious and means adding these bounds in a library is not just a theoretical breaking change, but one that will happen. The impl would be trivial.

[jonas-schievink]

I could also imagine implicit coercions from Box<A + B> to Box<A> and Box<B> to work quite well in general.

Tangentially related: #28326 and rust-lang/rfcs#1277

[steveklabnik]

/cc @rust-lang/libs

[steveklabnik]

Triage: trivial impls like this can be done via PR; please send one in if this is still needed, thanks!