Unusual request for a Sized bound

[alexcrichton]

In the code below, if type parameters are explicitly specified then the compilation succeeds, but if left to inference the compilation will fail.

use std::cell::Cell;
use std::sync::Arc;

pub trait PoolManager<C>: Send+Sync {}

struct InnerPool<C, M> {
    f: Option<C>,
}

pub struct Pool<C, M> where M: PoolManager<C> {
    helper_chan: Cell<()>,
    inner: Arc<InnerPool<C, M>>
}

impl<C, M> Pool<C, M>
        where C: Send + Sync, M: PoolManager<C> {
    pub fn new(manager: M) -> Pool<C, M> {
        let inner = Arc::new(InnerPool {
            f: None,
        });

        Pool {
            helper_chan: Cell::new(()),
            inner: inner,
        }
    }
}

#[cfg(bad)]
fn helper_task<C, M>(inner: Arc<InnerPool<C, M>>)
        where C: Send + Sync, M: PoolManager<C> {
    add_connection(&*inner);
}

#[cfg(good)]
fn helper_task<C, M>(inner: Arc<InnerPool<C, M>>)
        where C: Send + Sync, M: PoolManager<C> {
    add_connection::<C, M>(&*inner);
}

fn add_connection<C, M>(inner: &InnerPool<C, M>) where M: PoolManager<C> {
}

$ rustc lib.rs --crate-type lib --cfg bad
lib.rs:32:5: 32:19 error: the trait `core::kinds::Sized` is not implemented for the type `C`
lib.rs:32     add_connection(&*inner);
              ^~~~~~~~~~~~~~
lib.rs:32:5: 32:19 note: the trait `core::kinds::Sized` must be implemented because it is required by `add_connectio
n`
lib.rs:32     add_connection(&*inner);
              ^~~~~~~~~~~~~~
lib.rs:32:5: 32:19 error: the trait `core::kinds::Sized` is not implemented for the type `M`
lib.rs:32     add_connection(&*inner);
              ^~~~~~~~~~~~~~
lib.rs:32:5: 32:19 note: the trait `core::kinds::Sized` must be implemented because it is required by `add_connectio
n`
lib.rs:32     add_connection(&*inner);
              ^~~~~~~~~~~~~~
lib.rs:32:20: 32:27 error: mismatched types: expected `&InnerPool<C,M>`, found `&InnerPool<C,M>` (expected type para
meter, found type parameter)
lib.rs:32     add_connection(&*inner);
                             ^~~~~~~
error: aborting due to 3 previous errors
$ rustc lib.rs --crate-type lib --cfg good
... warnings

[alexcrichton]

This was also a recent regression, the code used to compile.

[aturon]

cc @nick29581

[nrc]

cc @nikomatsakis if this is a recent regression, I suspect more multi-dispatch issues

[nikomatsakis]

weird.

[nikomatsakis]

This seems to be the most interesting part:

lib.rs:32:20: 32:27 error: mismatched types: expected `&InnerPool<C,M>`, found `&InnerPool<C,M>` (expected type parameter, found type parameter)

[nikomatsakis]

Sounds like a missing substitution somewhere.

[nikomatsakis]

Possible dup of #18209.

[nikomatsakis]

Verified dup of #18209