Inference failure when a trait is changed to a marker trait

[scottmcm]

#![feature(marker_trait_attr)]

#[marker] // Remove this line and it works?!?
trait Foo<T> {}
impl Foo<u16> for u8 {}
impl Foo<[u8; 1]> for u8 {}
fn foo<T: Foo<U>, U>(_: T) -> U { unimplemented!() }

fn main() {
    let _: u16 = foo(0_u8);
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=120bf7cdd44e52d7a6d2280cdcdc6de6

It works if I remove the #[marker], but when it's a #[marker] trait it fails with

error[E0308]: mismatched types
  --> src/lib.rs:10:18
   |
10 |     let _: u16 = foo(0_u8);
   |                  ^^^^^^^^^ expected u16, found array of 1 elements
   |
   = note: expected type `u16`
              found type `[u8; 1]`

(I didn't think this property of a trait affected inference at all, so I'm very confused by this...)

[Aaron1011]

I ran into this with a slightly different example:

#![feature(marker_trait_attr)]

struct Wrapper<T>(T);

#[marker]
pub trait MyTrait {}

impl MyTrait for Wrapper<u32> {}
impl MyTrait for Wrapper<bool> {}

fn weird_call<T>(val: T) -> Wrapper<T> where Wrapper<T>: MyTrait {
    Wrapper(val)
}

fn main() {
    let a: Wrapper<u32> = weird_call(25);
}

with the following error:

error[E0308]: mismatched types
  --> src/main.rs:16:38
   |
16 |     let a: Wrapper<u32> = weird_call(25);
   |                                      ^^ expected `bool`, found integer

error[E0308]: mismatched types
  --> src/main.rs:16:27
   |
16 |     let a: Wrapper<u32> = weird_call(25);
   |            ------------   ^^^^^^^^^^^^^^ expected `u32`, found `bool`
   |            |
   |            expected due to this
   |
   = note: expected struct `Wrapper<u32>`
              found struct `Wrapper<bool>`

error: aborting due to 2 previous errors

Note that this example doesn't even rely on MyTrait being a marker trait - you can comment out #[marker], and it will compile.