E4221

Compiler diagnostic name: control_in_with_pattern_default.

A default expression in an or-pattern's with clause attempts to transfer control with return, break, continue, or raise. The default must produce a value locally and cannot leave its surrounding pattern.

Erroneous example

///|
pub fn value_or_zero(value : Int?) -> Int {
  match value {
    Some(x) | (None with x = { return 0 }) => x
  }
}

Suggestion

Use a plain value as the default and perform any control flow after the pattern has matched.

///|
pub fn value_or_zero(value : Int?) -> Int {
  match value {
    Some(x) | (None with x = 0) => x
  }
}