SI-7020 Determinism for pattern matcher warnings

Use LinkedHashSet for the DPLL algorithm for determistic
counter example generation.

Before, the test compiled with:

    [info] v2.10.2 => /Users/jason/usr/scala-v2.10.2-0-g60d462e
    test/files/neg/t7020.scala:3: warning: match may not be exhaustive.
    It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7))), List(_, _)
      List(5) match {
          ^
    test/files/neg/t7020.scala:10: warning: match may not be exhaustive.
    It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7))), List(_, _)
      List(5) match {
          ^
    test/files/neg/t7020.scala:17: warning: match may not be exhaustive.
    It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 6, 7)), _), List(1, _), List(2, _), List(4, _), List(5, _), List(6, _), List(7, _), List(??, _)
      List(5) match {
          ^
    test/files/neg/t7020.scala:24: warning: match may not be exhaustive.
    It would fail on the following input: List(_, _)
      List(5) match {
          ^

4 files changed, 53 insertions, 4 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/Solving.scala b/src/compiler/scala/tools/nsc/transform/patmat/Solving.scala
index 843f831ea1..ec66bf6f20 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/Solving.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/Solving.scala
@@ -208,15 +208,16 @@ trait Solving extends Logic {
             withLit(findModelFor(dropUnit(f, unitLit)), unitLit)
           case _ =>
             // partition symbols according to whether they appear in positive and/or negative literals
-            val pos = new mutable.HashSet[Sym]()
-            val neg = new mutable.HashSet[Sym]()
+            // SI-7020 Linked- for deterministic counter examples.
+            val pos = new mutable.LinkedHashSet[Sym]()
+            val neg = new mutable.LinkedHashSet[Sym]()
             f.foreach{_.foreach{ lit =>
               if (lit.pos) pos += lit.sym else neg += lit.sym
             }}
             // appearing in both positive and negative
-            val impures = pos intersect neg
+            val impures: mutable.LinkedHashSet[Sym] = pos intersect neg
             // appearing only in either positive/negative positions
-            val pures = (pos ++ neg) -- impures
+            val pures: mutable.LinkedHashSet[Sym] = (pos ++ neg) -- impures
 
             if (pures nonEmpty) {
               val pureSym = pures.head
diff --git a/test/files/neg/t7020.check b/test/files/neg/t7020.check
new file mode 100644
index 0000000000..a869b12363
--- /dev/null
+++ b/test/files/neg/t7020.check
@@ -0,0 +1,17 @@
+t7020.scala:3: error: match may not be exhaustive.
+It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7))), List(_, _)
+  List(5) match {
+      ^
+t7020.scala:10: error: match may not be exhaustive.
+It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7))), List(_, _)
+  List(5) match {
+      ^
+t7020.scala:17: error: match may not be exhaustive.
+It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7))), List(_, _)
+  List(5) match {
+      ^
+t7020.scala:24: error: match may not be exhaustive.
+It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7))), List(_, _)
+  List(5) match {
+      ^
+four errors found
diff --git a/test/files/neg/t7020.flags b/test/files/neg/t7020.flags
new file mode 100644
index 0000000000..e8fb65d50c
--- /dev/null
+++ b/test/files/neg/t7020.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/neg/t7020.scala b/test/files/neg/t7020.scala
new file mode 100644
index 0000000000..cc5421bab1
--- /dev/null
+++ b/test/files/neg/t7020.scala
@@ -0,0 +1,30 @@
+object Test {
+  // warning was non-deterministic
+  List(5) match {
+    case 1 :: Nil | 2 :: Nil  =>
+    case (x@(4 | 5 | 6)) :: Nil =>
+    case 7 :: Nil  =>
+    case Nil =>
+  }
+
+  List(5) match {
+    case 1 :: Nil | 2 :: Nil  =>
+    case (x@(4 | 5 | 6)) :: Nil =>
+    case 7 :: Nil  =>
+    case Nil =>
+  }
+
+  List(5) match {
+    case 1 :: Nil | 2 :: Nil  =>
+    case (x@(4 | 5 | 6)) :: Nil =>
+    case 7 :: Nil  =>
+    case Nil =>
+  }
+
+  List(5) match {
+    case 1 :: Nil | 2 :: Nil  =>
+    case (x@(4 | 5 | 6)) :: Nil =>
+    case 7 :: Nil  =>
+    case Nil =>
+  }
+}