enum GraphError<T> {
    Invalid(T),
}

trait Graph<T> {
    type Vertex;
    type Err = GraphError<Self::Vertex>;

    fn foo(&self) -> Result<(), Self::Err>;
}

// ----------------------

use std::result::Result as stdResult;

type Result<T> = stdResult<T, GraphError<usize>>;

impl<T> Graph<T> for SimpleGraph<T> {
    type Vertex = usize;

    fn foo(&self) -> Result<()> {
        unimplemented!();
    }
    // note: expected type `fn(&simple_graph::SimpleGraph<T>) -> std::result::Result<(), <SimpleGraph<T> as Graph<T>>::Err>`
    //          found type `fn(&simple_graph::SimpleGraph<T>) -> std::result::Result<(), GraphError<usize>>
}