Missing exhaustivity warning when pattern has a guard

[smarter]

The following code compiles without warning, even though the pattern isn't exhaustive:

object Main {
  def foo(x: Option[Int]): Int = x match {
    case Some(n) if n % 2 == 0 => n
    case None => 0
  }
  
  def main(args: Array[String]): Unit = println(foo(Some(1)))
}

It looks like we special cases guards for redundancy checks but not for exhaustivity checks (https://github.com/lampepfl/dotty/blob/85ab20fd874994634b909f0883760413f6e2ef12/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala#L812), but we need to do both to be safe.

See also previous discussions in scala/bug#5365

[liufengyun]

This is now fixed in d5f9047 .

I agree with Richard Bradley:

scala/bug#5365 (comment)

Richard Bradley (richard.bradley) said:
I don't think that example stands up to scrutiny. If g1 || g2 really is constant true, then rewrite your proposed:

x match {
  case p if g1 => ...
  case p if g2 => ...
}

to just:

x match {
  case p if g1 => ...
  case p => ...
}

This will a) allow the compiler to verify exhaustiveness and b) will help the reader to understand the code.