Confusing error message with boxed unsized types

[ftxqxd]

struct Foo<Sized? T>;

impl<Sized? T> Foo<T> {
    fn frob(self, bar: Box<T>) {}
}

fn main() {
    let foo: Foo<Num> = Foo;
    foo.frob(box 1i as Box<Num>);
}

yields the error

dstbug.rs:9:14: 9:32 error: mismatched types: expected `Box<core::num::Num>`, found `Box<core::num::Num>` (expected box, found box)
dstbug.rs:9     foo.frob(box 1i as Box<Num>);
                         ^~~~~~~~~~~~~~~~~~
error: aborting due to previous error

cc @nick29581

[nrc]

Hmm, this shouldn't be an error at all, let alone a confusing one. I expect this is due to us not unifying unsized types with an inference variable. I would expect a trait to unify to itself though. I'll have a poke around with this.

[nrc]

See also #16918 which has a related problem with unifying unsized type variables

[ftxqxd]

Hmm. #17178 has been closed, but this still isn’t fixed, so perhaps it’s not related to unifying unsized types in type inference. An interesting thing is that the error message (expected box, found box) seems to suggest that it’s not the traits that differ, but the box: replacing as Box<Num> with as Box<Float> gives a message that correctly states that the traits differ.

[bkoropoff]

The example builds for me with the latest master.

[ftxqxd]

Ah, it does too. It looks like the playpen is running a very outdated version of rustc…

I guess this is fixed, then! Closing.