GAT's arent typechecked, and can cause UB #68641
Description
It looks like Generic Associated Types aren't validated on whether the specified type on the impl-side indeed implements the traits as written down in the definition of the associated type.
The following program shows a use-after-free of a String:
#![feature(generic_associated_types)] trait UnsafeCopy { type Item<'a>: Copy; fn copy<'a>(item: &Self::Item<'a>) -> Self::Item<'a> { *item } } impl <T> UnsafeCopy for T { type Item<'a> = T; } fn main() { let mut s = String::from("Hello world!"); let copy = String::copy(&s); // Do we indeed point to the samme memory? assert!(s.as_ptr() == copy.as_ptr()); // Any use of `copy` is certeinly UB after this drop(s); // UB UB UB UB UB!! println!("{}", copy); }