Bad ambigous error typechecking

[philberty]

we're also running into this with this code example - I suspect the fix would be the same and there is something wrong when it comes to references/pointers and mutability:

#![feature(intrinsics)]

#[lang = "sized"]
trait Sized {}

pub struct VaListImpl<'f>;

mod sealed_trait {
    pub trait VaArgSafe {}
}

impl<T> sealed_trait::VaArgSafe for *mut T {}
impl<T> sealed_trait::VaArgSafe for *const T {}

impl<'f> VaListImpl<'f> {
    /// Advance to the next arg.
    #[inline]
    pub unsafe fn arg<T: sealed_trait::VaArgSafe>(&mut self) {
        va_arg2(self);
    }
}

fn va_arg2<T: sealed_trait::VaArgSafe>(ap: &mut VaListImpl<'_>) {}

Originally posted by @CohenArthur in #3032 (comment)

[powerboat9]

It looks like rustc might also find an ambiguity here

[CohenArthur]

if rustc finds an ambiguity, then my testcase was reduced too far. there was definitely an issue with the original code which was taken directly from core when trying to compile it. I can try and get an old gccrs build to see what it was exactly