Incorrect "error: private type in exported type signature"

[Diggsey]

The following code will fail to compile:

pub trait Test {
    type Result;
}

mod Foo {
    pub struct Ty;
}

struct Bar;

impl Test for Foo::Ty {
    type Result = Bar;
}

fn main() {
}

When in fact no private types are exported.

[alexcrichton]

This is actually currently working as intended I believe. The definition of a "public signature" is that the type Ty is marked pub, so it's considered a public type subject to the privacy rules. You can find more details in the associated RFC

[Diggsey]

If so, that's disappointing. It seriously hinders the ability to make some patterns modular.

In the example, there's no way to access Foo::Ty from the outer scope without making it public, and yet to do so forces other types to be exported from that scope!