tests/ui/specialization/specialization-default-projection.rs

   1 #![feature(specialization)] //~ WARN the feature `specialization` is incomplete
   2
   3 // Make sure we can't project defaulted associated types
   4
   5 trait Foo {
   6     type Assoc;
   7 }
   8
   9 impl<T> Foo for T {
  10     default type Assoc = ();
  11 }
  12
  13 impl Foo for u8 {
  14     type Assoc = String;
  15 }
  16
  17 fn generic<T>() -> <T as Foo>::Assoc {
  18     // `T` could be some downstream crate type that specializes (or,
  19     // for that matter, `u8`).
  20
  21     () //~ ERROR mismatched types
  22 }
  23
  24 fn monomorphic() -> () {
  25     // Even though we know that `()` is not specialized in a
  26     // downstream crate, typeck refuses to project here.
  27
  28     generic::<()>() //~ ERROR mismatched types
  29 }
  30
  31 fn main() {
  32     // No error here, we CAN project from `u8`, as there is no `default`
  33     // in that impl.
  34     let s: String = generic::<u8>();
  35     println!("{}", s); // bad news if this all compiles
  36 }