Fix #8708: treat guard as false

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -812,7 +812,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
     if (!exhaustivityCheckable(sel)) return
 
     val patternSpace = cases.map({ x =>
-      val space = project(x.pat)
+      val space = if (x.guard.isEmpty) project(x.pat) else Empty
       debug.println(s"${x.pat.show} ====> ${show(space)}")
       space
     }).reduce((a, b) => Or(List(a, b)))

tests/patmat/i8708.check

@@ -0,0 +1 @@
+2: Pattern Match Exhaustivity: Some(_)

tests/patmat/i8708.scala

@@ -0,0 +1,8 @@
+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)))
+}

tests/patmat/patmatexhaust.check

@@ -2,6 +2,7 @@
 11: Pattern Match Exhaustivity: Bar(_)
 23: Pattern Match Exhaustivity: (Qult(), Qult()), (Kult(_), Kult(_))
 49: Pattern Match Exhaustivity: _: Gp
+53: Pattern Match Exhaustivity: _: Gp
 59: Pattern Match Exhaustivity: Nil
 75: Pattern Match Exhaustivity: _: B
 87: Pattern Match Exhaustivity: _: C1

tests/patmat/t7631.check

@@ -1 +1 @@
-8: Pattern Match Exhaustivity: TestB()
+8: Pattern Match Exhaustivity: _: Test

tests/patmat/virtpatmat_apply.check

@@ -1 +1 @@
-2: Pattern Match Exhaustivity: List(_)
+2: Pattern Match Exhaustivity: List(_, _: _*)