| // Regression test for #46989: |
| // |
| // In the move to universes, this test started passing. |
| // It is not necessarily WRONG to do so, but it was a bit |
| // surprising. The reason that it passed is that when we were |
| // asked to prove that |
| // |
| // for<'a> fn(&'a i32): Foo |
| // |
| // we were able to use the impl below to prove |
| // |
| // fn(&'empty i32): Foo |
| // |
| // and then we were able to prove that |
| // |
| // fn(&'empty i32) = for<'a> fn(&'a i32) |
| // |
| // This last fact is somewhat surprising, but essentially "falls out" |
| // from handling variance correctly. In particular, consider the subtyping |
| // relations. First: |
| // |
| // fn(&'empty i32) <: for<'a> fn(&'a i32) |
| // |
| // This holds because -- intuitively -- a fn that takes a reference but doesn't use |
| // it can be given a reference with any lifetime. Similarly, the opposite direction: |
| // |
| // for<'a> fn(&'a i32) <: fn(&'empty i32) |
| // |
| // holds because 'a can be instantiated to 'empty. |
| |
| trait Foo {} |
| |
| impl<A> Foo for fn(A) {} |
| |
| fn assert_foo<T: Foo>() {} |
| |
| fn main() { |
| assert_foo::<fn(&i32)>(); |
| //~^ ERROR implementation of `Foo` is not general enough |
| } |
