XO0303: refutable pattern where an irrefutable one is required

let and for bind every value they get, so their patterns must match every value of the type, such as (a, b) for a tuple or P{x, y} for a struct. A pattern that can fail, such as Some(v), needs a construct that says what happens otherwise: if let, match, ??, or ?. The message names a value that is not covered. See core 3.5.

Example

fn f(o: Int?) -> Int {
  let Some(v) = o
  v
}

Fix

fn f(o: Int?) -> Int {
  if let Some(v) = o { v } else { 0 }
}

Run xo explain XO0303 for this text in a terminal, or see section 10.6 of the specification.