[E0282] Cannot infer the result type of map

[nullpo-head]

If there is a type T for which FromStr and FromIterator are implemented and is used in an option as Vec<T> or Option<Vec<T>>, the build will fail by the following error.

error[E0282]: type annotations needed
   --> tests/issues.rs:140:18
    |
140 |         opt_vec: Vec<u16>,
    |         ---------^^^^^^^^
    |         |        |
    |         |        cannot infer type for type parameter `B` declared on the associated function `map`
    |         this method call resolves to `T`

The whole example code is

    struct U16ish;
    impl FromStr for U16ish {
        type Err = ();
        fn from_str(_: &str) -> Result<Self, Self::Err> {
            unimplemented!()
        }
    }
    impl<'a> FromIterator<&'a U16ish> for Vec<u16> {
        fn from_iter<T: IntoIterator<Item = &'a U16ish>>(_: T) -> Self {
            unimplemented!()
        }
    }

    #[derive(StructOpt, Debug)]
    struct Opt {
        opt_vec: Vec<u16>,
        #[structopt(long)]
        opt_opt_vec: Option<Vec<u16>>,
    }

The reason of this build failure is the type of the map in the expanded code becomes ambiguous due to U16ish.

    fn from_clap(matches: &::structopt::clap::ArgMatches) -> Self {
        Opt {
            opt_vec: matches.values_of("opt_vec").map_or_else(Vec::new, |v| {
                v.map(|s| ::std::str::FromStr::from_str(s).unwrap())
                    .collect()
            }),
        }
    }

This map has two possible types, thus the type inference needs an annotation and the build fails.

// u16
            opt_vec: matches.values_of("opt_vec").map_or_else(Vec::new, |v| {
                v.map::<u16, _>(|s| ::std::str::FromStr::from_str(s).unwrap())
                    .collect()
            }),
// Or, U16ish is also possible
            opt_vec: matches.values_of("opt_vec").map_or_else(Vec::new, |v| {
                v.map::<U16ish, _>(|s| ::std::str::FromStr::from_str(s).unwrap())
                    .collect()
            }),

This situation may sound unlikely at first, but it actually happens when you use Structopt with Indicatif crate on Windows. Indicatif defines Utf16Char on Windows and it causes this error.

I've also submitted a PR #491 to fix this. Thanks!