Type inference fails in light of generic type aliases with default parameter.

[glandium]

Consider the following source:

pub struct Foo<A: Default, B: Default>(A, B);

impl<A: Default, B: Default> Foo<A, B> {
    fn new(a: A) -> Self { Foo(a, B::default()) }
}

pub type Bar<A, B=usize> = Foo<A, B>;

fn main() {
    let bar = Bar::new(42usize);
}

This fails to build with:

error[E0283]: type annotations required: cannot resolve `_: std::default::Default`
  --> src/main.rs:10:15
   |
10 |     let bar = Bar::new(42usize);
   |               ^^^^^^^^
   |
note: required by `<Foo<A, B>>::new`
  --> src/main.rs:4:5
   |
4  |     fn new(a: A) -> Self { Foo(a, B::default()) }
   |     ^^^^^^^^^^^^^^^^^^^^

IOW, it does want Bar::<usize>::new instead of Bar::new.

pub type Bar<A> = Foo<A, usize> also does Bar::new work.

[glandium]

Huh, scrap that, it's not even type inference, because the following compiles:

pub struct Foo<A: Default, B: Default>(A, B);

impl<A: Default, B: Default> Foo<A, B> {
    fn new(a: A) -> Self { Foo(a, B::default()) }
}

pub type Bar<A, B=usize> = Foo<A, B>;

fn main() {
    let bar = Bar::<_>::new(42usize);
}

I don't know what the hell the problem is.

[glandium]

As mentioned by talchas on irc, this doesn't even need type aliases:

pub struct Foo<A: Default, B = usize>(A, B);

impl<A: Default, B> Foo<A, B> {
    fn new(a: A) -> Self { Foo(a, panic!()) }
}

fn main() {
    let bar = Foo::new(42usize);
}

Fails with:

error[E0282]: type annotations needed
 --> src/main.rs:8:15
  |
8 |     let bar = Foo::new(42usize);
  |         ---   ^^^^^^^^ cannot infer type for `B`
  |         |
  |         consider giving `bar` a type

In this example, Foo::<_>::new compiles, which is weird, considering the error is complaining about B.