Unexpected type argument in closure

[xldenis]

the following program crashes

extern crate creusot_contracts;
use creusot_contracts::*;
struct X<T>(Vec<Option<T>>);

impl<T> X<T> {
    pub fn foo() -> Self {
        let mut v: Vec<_> = (0..10).map_inv(|_, _| None).collect();
        X(v)
    }
}