trait的冲突实现

试图回答时 this question

trait MyTrait {
    type A;
    type B;
}

trait WithFoo: MyTrait {
    fn foo(a: Self::A) -> Self::B;
}

impl<T, U: MyTrait<A = T, B = T>> WithFoo for U {
    fn foo(a: T) -> T {
        a
    }
}

struct S1;

impl MyTrait for S1 {
    type A = u32;
    type B = f32;
}

impl WithFoo for S1 {
    fn foo<T>(a: Self::A) -> Self::B {
        a as f32
    }
}

struct S2;

impl MyTrait for S2 {
    type A = u32;
    type B = u32;
}

fn main() {
    S1::foo(42);
    S2::foo(42);
}

playground

编译失败,出现以下错误:

error[E0119]: conflicting implementations of trait `WithFoo` for type `S1`:
  --> src/main.rs:23:1
   |
10 | impl<T, U: MyTrait<A = T, B = T>> WithFoo for U {
   | ----------------------------------------------- first implementation here
...
23 | impl WithFoo for S1 {
   | ^^^^^^^^^^^^^^^^^^^ conflicting implementation for `S1`

根据 these answers S1 impl 当编译器不能确定某个未来的毯子 执行 会导致 第一节

但是在这种情况下,不可能 第一节 MyTrait 具有 A == B 因为它已经实现了 我的特质 具有 A != B . 这个错误是当前编译器实现的一个限制,可能在以后的某个时候被解除,还是我还缺少其他东西?