Reordering type parameters leads to wrong inference

[Schaeff]

This code compiles:

enum Result<T, E> {
    Ok(T),
    Err(E)
}

let<T, E, U> map: Result<T, E>, (T -> U) -> Result<U, E> = |result, f| match result {
    Result::Ok(v) => Result::Ok(f(v)),
    Result::Err(e) => Result::Err(e)
};

But if we change this

-let<T, E, U>
+let<T, U, E>

We get this error:

error: Inferred type scheme for symbol std::result::map does not match the declared type.
Inferred: let<T1, T2, T3> std::result::map: std::result::Result<T1, T2>, (T1 -> T3) -> std::result::Result<T3, T2>
Declared: let<T, U, E> std::result::map: std::result::Result<T, E>, (T -> U) -> std::result::Result<U, E>

It seems like type parameters must be ordered in the same order they appear in the type for inference to work.

[chriseth]

It's a duplicate of #1305 (but let me close that one).

In general, I'm also considering not making it an error if the declared type is not the most generic possible, just check that it's compatible with the code...