gistfile1.rs

type Date = chrono::Date<chrono::Local>;

pub struct DateIterator {
    start: Date,
    end: Date
}

impl Iterator for DateIterator {
    // ... not important
}

impl DateIterator {
    pub fn by_months<'a>(self) -> Groups<'a, u32, DateIterator, fn(&Date) -> u32> {
        fn month(d: &Date) -> u32 { d.month() }

        self.group_by_lazy(month).into_iter() // <-- ERROR
    }
} 

// expected `itertools::groupbylazy::Groups<'a, u32, iter::DateIterator, fn(&chrono::date::Date<chrono::offset::local::Local>) -> u32>`,
//  found `itertools::groupbylazy::Groups<'_, u32, iter::DateIterator, fn(&chrono::date::Date<chrono::offset::local::Local>) -> u32 {iter::DateIterator::by_months::month}>`
// (expected fn pointer,
//    found fn item) [E0308]