ch18-03: Guarded match arm exhaustivness clarification

[bravequickcleverfibreyarn]

As per experimentation shows, match arms exhaustivness is checked even when arms use match guard.

This simple sample

pub fn test() {
    match Some(5) {
        Some(50) => println!("Got it"),
        Some(_n) if true  => println!("Exhausted"),
        None => (),
    }
}

produces output

Notable observations are

• Arm Some(_n) if true => println!("Exhausted") catches already any other Some other than Some(50).
• After None => (), change to _ => (),, compilation produces no error.

---

Obviously compiler does no heuristics on possible results of match guard. But it still acts as if makes strong check on arms exhaustivness.

[chriskrycho]

Thanks, but the original text here is correct, it is just not exhaustively correct! That's an important balance we have to walk given how long the book already is. Going to close this accordingly.

[bravequickcleverfibreyarn]

• I see it merged not closed.
• I agree original text is correct. Maybe just sense is little concealed.

[chriskrycho]

Oh good grief, I hit the wrong button again. 😂 GitHub’s UI is going to be the death of me! Thanks again!