trait ToNum {
    type Num;
}
trait Bar {
    type Ty;
}
struct Gen<const B: bool>;
struct Int<const B: bool>;

impl<const B: bool> ToNum for Gen<{B}> { type Num = Int<B>; }
//impl<const B: bool> ToNum for Gen<B> { type Num = Int<B>; }

impl Bar for Int<true> {
    type Ty = i32;
}
impl Bar for Int<false> {
    type Ty = f32;
}

type A = <<Gen<true> as ToNum>
            ::Num as Bar>
            ::Ty;

fn main() {
    let x = A::default();
    // If use `Gen<B>`, x: i32.
    // but use `Gen<{B}>`, x: {unknown}
}