Merge pull request scala#4670 from retronym/ticket/9422

SI-9422 Fix incorrect constant propagation

Author: lrytz

src/compiler/scala/tools/nsc/backend/opt/ConstantOptimization.scala

@@ -170,9 +170,11 @@ abstract class ConstantOptimization extends SubComponent {
         // out all the possibilities
         case Impossible(possible2) => (possible -- possible2).nonEmpty
       })
-      def mightNotEqual(other: Contents): Boolean = (this ne other) && (other match {
-        // two Possibles might not be equal if either has possible members that the other doesn't
-        case Possible(possible2) => (possible -- possible2).nonEmpty || (possible2 -- possible).nonEmpty
+      def mightNotEqual(other: Contents): Boolean = (other match {
+        case Possible(possible2) =>
+          // two Possibles must equal if each is known to be of the same, single value
+          val mustEqual = possible.size == 1 && possible == possible2
+          !mustEqual
         case Impossible(_) => true
       })
     }

test/files/run/t9422.scala

@@ -0,0 +1,11 @@
+class Test(val x: Long) {
+  def sameDirection(y: Long): Boolean =
+    (y == 0 || x == 0 || ((y > 0) == (x > 0)))
+}
+ 
+object Test {
+  def main(args: Array[String]) {
+    val b = new Test(1L)
+    assert(!b.sameDirection(-1L))
+  }
+}