Can create iterator adapter that fulfills `Iterator<Item=T>` and `Iterator<Item=Option<T>>`

[japaric]

STR

Example from libcore

#![crate_type = "lib"]
#![feature(associated_types)]
#![no_implicit_prelude]

use std::option::Option::{None, Some, mod};
use std::vec::Vec;

trait Iterator {
    type Item;

    fn next(&mut self) -> Option<Self::Item>;
}

fn is_iterator_of<A, I: Iterator<Item=A>>(_: &I) {}

struct Adapter<I> {
    iter: I,
    found_none: bool,
}

impl<T, I> Iterator for Adapter<I> where I: Iterator<Item=Option<T>> {
    type Item = T;

    fn next(&mut self) -> Option<T> {
        loop {}
    }
}

fn test_adapter<T, I: Iterator<Item=Option<T>>>(it: I) {
    is_iterator_of::<Option<T>, _>(&it);  // Sanity check
    let adapter = Adapter { iter: it, found_none: false };
    is_iterator_of::<T, _>(&adapter); // OK
    is_iterator_of::<Option<T>, _>(&adapter); // <-- This shouldn't be accepted
}

Version

84f5ad8

cc @nikomatsakis