SI-9422 Fix incorrect constant propagation

The ConstantOptimization phase uses abstract interpretation
to track what is known about values, and then to use this information
to optimize away tests with a statically known result.

Constant propagation was added under -optimize in Scala 2.11.0-M3, in
PR #2214.

For example, we might know that a variable must hold one of a set
of values (`Possible`). Or, we could track that it must *not*
be of of a set of value (`Impossible`).

The test case in the bug report was enough to create comparison:

    v1 == v2 // where V1 = Possible(Set(true, false))
             //       V2 = Possible(Set(true, false))

This test was considered to be always true, due to a bug in
`Possible#mightNotEqual`. The only time we can be sure that
`Possible(p1) mightNotEquals Possible(p2)` is if
`p1` and `p2` are the same singleton set. This commit changes
this method to implement this logic.

The starting assumption for all values is currently
`Impossible(Set())`, although it would also be reasonable to represent
an unknown boolean variable as `Possible(Set(true, false))`, given
the finite and small domain.

I tried to change the starting assumption for boolean locals in
exactly this manner, and that brings the bug into sharp focus.

Under this patch:

    https://github.com/retronym/scala/commit/e564fe522d

This code:

   def test(a: Boolean, b: Boolean) = a == b

Compiles to:

  public boolean test(boolean, boolean);
    Code:
       0: iconst_1
       1: ireturn

Note: the enclosed test case does not list `-optimize` in a `.flags`
file, I'm relying on that being passed in by the validation build.
I've tested locally with that option, though.

1 files changed, 5 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/opt/ConstantOptimization.scala b/src/compiler/scala/tools/nsc/backend/opt/ConstantOptimization.scala
index 0e6ee76eb2..fb1799e092 100644
--- a/src/compiler/scala/tools/nsc/backend/opt/ConstantOptimization.scala
+++ b/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
       })
     }