src/test/ui/hrtb/issue-46989.rs

   1 // Regression test for #46989:
   2 //
   3 // In the move to universes, this test started passing.
   4 // It is not necessarily WRONG to do so, but it was a bit
   5 // surprising. The reason that it passed is that when we were
   6 // asked to prove that
   7 //
   8 //     for<'a> fn(&'a i32): Foo
   9 //
  10 // we were able to use the impl below to prove
  11 //
  12 //     fn(&'empty i32): Foo
  13 //
  14 // and then we were able to prove that
  15 //
  16 //     fn(&'empty i32) = for<'a> fn(&'a i32)
  17 //
  18 // This last fact is somewhat surprising, but essentially "falls out"
  19 // from handling variance correctly. In particular, consider the subtyping
  20 // relations. First:
  21 //
  22 //     fn(&'empty i32) <: for<'a> fn(&'a i32)
  23 //
  24 // This holds because -- intuitively -- a fn that takes a reference but doesn't use
  25 // it can be given a reference with any lifetime. Similarly, the opposite direction:
  26 //
  27 //     for<'a> fn(&'a i32) <: fn(&'empty i32)
  28 //
  29 // holds because 'a can be instantiated to 'empty.
  30
  31 trait Foo {}
  32
  33 impl<A> Foo for fn(A) {}
  34
  35 fn assert_foo<T: Foo>() {}
  36
  37 fn main() {
  38     assert_foo::<fn(&i32)>();
  39     //~^ ERROR implementation of `Foo` is not general enough
  40 }