Region parameterization inference does not consider inheritance

[erickt]

See this code:

pub trait Map<K, V> {
    pure fn f(&self) -> Option<&self/V>;
}

pub trait MutableMap<K, V>: Map<K, V> {
}

fn main() { }

It fails with:

foo.rs:7:28: 7:37 error: to use region types here, the containing type must be declared with a region bound
foo.rs:7 pub trait MutableMap<K, V>: Map<K, V> {
                                     ^~~~~~~~~
error: aborting due to previous error

I'm not quite sure what to do in this case. Is this an error? If not, how do I pass a region bound to a parent trait?

[nikomatsakis]

This is a failure of region inference.

[nikomatsakis]

That is, region parameterization inference. It is not aware of supertraits. However, I am reluctant to try and fix it because we plan to make all these region parameters more explicit. Also, I think that in this case the trait should not be region-parameterized anyhow.

[nikomatsakis]

(So actually there are two bugs, in a way)

[nikomatsakis]

The first bug should be fixed now (dup of #5225), the second bug (region parameterization inference doesn't consider trait inheritance) I don't plan to fix since I plan to remove RPI.

[nikomatsakis]

On second thought, some from of RPI will remain (variance inference), so I am re-opening and re-titling.

[nikomatsakis]

Never mind, dup of #3598.