Custom Option type leads to error in unrelated function

//This code gives errors (see end of paste). The error does not occur if the enum Opt<T> is commented out, or if the 'case-values' Some and None are mangled to some other names to (seemingly) avoid conflict with the same members of the Option<T> type from the core library.
//How does this error occur? Why does the map function think that I want to use this custom type? Its name doesn't even conflict with the Option<T> type, but I wouldn't expect that that would be a problem anyway, since I assumed that the type of vector.iter() (I assume that this is where the error is) wouldn't be affected by any shadowing in the current rust file.

enum Opt<T> {
    Some(T),
    None
}

fn map<T, U>(vector: &[T], fun: |v: &T| -> U) -> Vec<U> {
    let mut acc = Vec::new();
    for elem in vector.iter() {
        acc.push(fun(elem));
    }
    return acc;
}

fn main() {
}

// Errors:

$ rustc debug.rs
debug.rs:9:5: 12:11 error: mismatched types: expected `core::option::Option<&T>` but found `Opt<<generic #221>>` (expected enum core::option::Option but found enum Opt)
debug.rs:9     for elem in vector.iter() {
debug.rs:10         acc.push(fun(elem));
debug.rs:11     }
debug.rs:12     return acc;
debug.rs:9:5: 12:11 error: mismatched types: expected `core::option::Option<&T>` but found `Opt<<generic #222>>` (expected enum core::option::Option but found enum Opt)
debug.rs:9     for elem in vector.iter() {
debug.rs:10         acc.push(fun(elem));
debug.rs:11     }
debug.rs:12     return acc;
error: aborting due to 2 previous errors