Type inference should success with generic trait and placeholder

[mu001999]

Here's an example (play):

trait Tr<Ctx> {
    fn foo();
}

trait DefaultCtx {}

impl<T: Default, Ctx: DefaultCtx> Tr<Ctx> for T {
    fn foo() {}
}

struct OneDefaultCtx;

impl DefaultCtx for OneDefaultCtx {}

fn main() {
    <i32 as Tr<_>>::foo();
}

The context type Ctx cannot be inferred even though:

1. i32 implements Default
2. There's at least one type (OneDefaultCtx) that implements DefaultCtx
3. The trait method foo() doesn't use Ctx in its signature

Is this inability to infer Ctx an intentional design choice in Rust (perhaps due to coherence rules, ambiguity, or other language design principles), or is it a current limitation of the compiler's type inference that might be improved in the future?