ch18-03: Guarded match arm exhaustivness clarification

[deep-outcome]

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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.

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