With min_specialization enabled, an incomplete impl for a non-static type will delegate method calls to a less-specific impl with a 'static bound

[fleabitdev]

Rustc version: 1.50.0-nightly (2020-11-25 b48cafd9eb658b5d7401), tested on the playground.

#![feature(min_specialization)]

use std::any::Any;

pub trait Tr {
    fn method(self) -> Box<dyn Any + 'static>;
}

impl<T: Any + 'static> Tr for T {
    fn method(self) -> Box<dyn Any + 'static> {
        Box::new(self)
    }
}

impl<'a> Tr for &'a i32 {
}

fn promote_to_static<'a>(i: &'a i32) -> &'static i32 {
    *i.method().downcast().unwrap()
}

struct Wrapper<'a>(&'a i32);

impl<'a> Tr for Wrapper<'a> {
}

fn promote_to_static_2<'a>(w: Wrapper<'a>) -> Wrapper<'static> {
    *w.method().downcast().unwrap()
}

fn main() {
    let i = Box::new(100_i32);
    let static_i: &'static i32 = promote_to_static(&*i);
    drop(i);
    println!("{}", *static_i);
    
    let j = Box::new(200_i32);
    let static_w: Wrapper<'static> = promote_to_static_2(Wrapper(&*j));
    drop(j);
    println!("{}", *static_w.0);
}

This bug also occurs when:

• The trait contains two methods, method_0 and method_1
• Both of those methods are implemented in the blanket impl
• Only method_0 is overridden for &i32
• We invoke method_1 on an &i32

I included the promote_to_static_2 function to demonstrate that this seems to be unrelated to autoderef; Wrapper does not implement Deref.

[voidc]

The given example does not compile with Rust 1.70.0-nightly (ec2f40c 2023-03-30). It produces the following error:

error[E0477]: the type `&'a i32` does not fulfill the required lifetime
  |
  = note: type must satisfy the static lifetime

error[E0477]: the type `Wrapper<'a>` does not fulfill the required lifetime
  |
  = note: type must satisfy the static lifetime

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=abcf336e6e3914ed4c9bce7952eaffa6

Therefore, I would propose to no longer label this issue as unsound but as a diagnostics issue.

[matthewjasper]

The error should point to some actual code though.