Wrong error message when using Self in some places

[bombless]

fn foo(_: Self) {}

fn main() {}

<anon>:1:11: 1:15 error: use of undeclared type name `Self`
<anon>:1 fn foo(_: Self) {}
                   ^~~~

Self is a keyword, this error message indicates that it can be used to define type, which is misleading.

[XuefengWu]

There is a similar issue: #12796 file.

[bombless]

No, that's a different issue.
At the time that issue was filed, Self was not a keyword, just a confusing type. That said, at the time that issue was reported, it's fine to say the compiler was implemented properly.
But my issue is about wrong error message, not bad(confusing) error message.

[XuefengWu]

Do you mean Self is not a special ident now?
I found the example code can be compiled,

pub trait Square {
    fn square(x: Self) -> Self;
}

and there is excluding self type code。

pub fn parse_ident_or_self_type(&mut self) -> PResult<ast::Ident> {
        if self.is_self_type_ident() {
            self.expect_self_type_ident()
        } else {
            self.parse_ident()
        }
    }

https://github.com/XuefengWu/rust/blob/master/src/libsyntax/parse/parser.rs#L558-L564

[bombless]

My point is that we are in different situation now.
See #22158 and notice that it happened after the time the issue we discuss was filed.

[Manishearth]

Actually, IIRC we're moving towards a direction where Self can be used everywhere in a trait or impl context. I'm not sure though. @pnkfelix any idea?

[bombless]

Yes we can use it in traits or impls.

trait T where Self: Sized {}

is totally legal for instance.

But Self inside global functions can not be explained if I understand correctly.

[Manishearth]

Oh, sorry, this was a global fn. Makes sense.