marker_trait_attr: overlapping impls can cause compilation failures

[lcnr]

#![feature(marker_trait_attr)]

#[marker]
pub trait F {}
impl<T> F for T where T: Copy {}
impl<T> F for T where T: 'static {}

results in

error[E0310]: the parameter type `T` may not live long enough
 --> src/lib.rs:5:9
  |
5 | impl<T> F for T where T: Copy {}
  |      -  ^ ...so that the type `T` will meet its required lifetime bounds
  |      |
  |      help: consider adding an explicit lifetime bound...: `T: 'static`

This should not happen

[lcnr]

taken from #29864 (comment)