Generic parameters can have defaults

[philberty]

For example this test case: https://github.com/rust-lang/rust/blob/master/src/test/ui/generics/generic-impl-less-params-with-defaults.rs

[bjorn3]

Defaults are not very well supported in rustc. Tyoe inference doesn't fall back to param defaults I believe.

[philberty]

Do you have an example @bjorn3? That is really interesting to know.

[bjorn3]

struct Box<T, A: Default = ()>(T, A);

impl<T, A: Default> Box<T, A> {
    fn new(x: T) -> Self {
        Box(x, Default::default())
    }
}

fn main() {
    Box::new(1);
}

error[E0283]: type annotations needed
  --> src/main.rs:10:5
   |
4  |     fn new(x: T) -> Self {
   |     -------------------- required by `Box::<T, A>::new`
...
10 |     Box::new(1);
   |     ^^^^^^^^ cannot infer type for type parameter `A`
   |
   = note: cannot satisfy `_: Default`

error: aborting due to previous error

[philberty]

Thanks!

[Soveu]

Some other example with explanation