SI-7369 Avoid spurious unreachable warnings in patterns

Unreachability analysis draws on the enumerated domain of types
(e.g sealed subclasses + null, or true/false), and also looks at all
stable identifier patterns tested for equality against the same 'slot'
in a pattern.

It was drawing the wrong conclusions about stable identifier patterns.
Unlike the domain constants, two such values may hold the same value,
so we can't assume that matching X precludes matching Y in the same
slot in a subsequent case.

For example:

    val X: Boolean = true; val Y: Boolean = true
    def m1(t1: Tuple1[Boolean]) = t1 match {
      case Tuple1(true) =>
      case Tuple1(false) =>
      case Tuple1(false) => // correctly unreachable
    }
    def m2(t1: Tuple1[Boolean]) = t1 match {
      case Tuple1(X) =>
      case Tuple1(Y) => // spurious unreachable warning
    }

    //
    // Before
    //
    reachability, vars:
    V2: Boolean ::= true | false// Set(false, Y, X, true) // = x1._1
    V1: (Boolean,) ::= null | ... // = x1
    equality axioms:
      V2=true#4  \/ V2=false#5  /\
     -V2=false#5 \/  -V2=Y#3    /\
     -V2=false#5 \/  -V2=X#2    /\
     -V2=false#5 \/ -V2=true#4  /\
       -V2=Y#3   \/  -V2=X#2    /\
       -V2=Y#3   \/ -V2=true#4  /\
       -V2=X#2   \/ -V2=true#4

    //
    // After
    //
    reachability, vars:
    V2: Boolean ::= true | false// Set(false, Y, X, true) // = x1._1
    V1: (Boolean,) ::= null | ... // = x1
    equality axioms:
      V2=true#4  \/ V2=false#5  /\
     -V2=false#5 \/ -V2=true#4

5 files changed, 95 insertions, 0 deletions

diff --git a/test/files/neg/t7369.check b/test/files/neg/t7369.check
new file mode 100644
index 0000000000..4f101e145a
--- /dev/null
+++ b/test/files/neg/t7369.check
@@ -0,0 +1,13 @@
+t7369.scala:6: error: unreachable code
+    case Tuple1(X) => // unreachable
+                   ^
+t7369.scala:13: error: unreachable code
+    case Tuple1(true) => // unreachable
+                      ^
+t7369.scala:31: error: unreachable code
+    case Tuple1(X) => // unreachable
+                   ^
+t7369.scala:40: error: unreachable code
+    case Tuple1(null) => // unreachable
+                      ^
+four errors found
diff --git a/test/files/neg/t7369.flags b/test/files/neg/t7369.flags
new file mode 100644
index 0000000000..e8fb65d50c
--- /dev/null
+++ b/test/files/neg/t7369.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/neg/t7369.scala b/test/files/neg/t7369.scala
new file mode 100644
index 0000000000..87ddfe98b7
--- /dev/null
+++ b/test/files/neg/t7369.scala
@@ -0,0 +1,43 @@
+object Test {
+  val X, Y = true
+  (null: Tuple1[Boolean]) match {
+    case Tuple1(X) =>
+    case Tuple1(Y) =>
+    case Tuple1(X) => // unreachable
+    case _      =>
+  }
+
+  (null: Tuple1[Boolean]) match {
+    case Tuple1(true) =>
+    case Tuple1(false) =>
+    case Tuple1(true) => // unreachable
+    case _      =>
+  }
+}
+
+
+sealed abstract class B;
+case object True extends B;
+case object False extends B;
+
+object Test2 {
+
+  val X: B = True
+  val Y: B = False
+
+  (null: Tuple1[B]) match {
+    case Tuple1(X) =>
+    case Tuple1(Y) =>
+    case Tuple1(X) => // unreachable
+    case _      =>
+  }
+}
+
+object Test3 {
+  (null: Tuple1[B]) match {
+    case Tuple1(null) =>
+    case Tuple1(True) =>
+    case Tuple1(null) => // unreachable
+    case _      =>
+  }
+}
diff --git a/test/files/pos/t7369.flags b/test/files/pos/t7369.flags
new file mode 100644
index 0000000000..85d8eb2ba2
--- /dev/null
+++ b/test/files/pos/t7369.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
diff --git a/test/files/pos/t7369.scala b/test/files/pos/t7369.scala
new file mode 100644
index 0000000000..2f31c93d29
--- /dev/null
+++ b/test/files/pos/t7369.scala
@@ -0,0 +1,37 @@
+object Test {
+  val X, Y = true
+  (null: Tuple1[Boolean]) match {
+    case Tuple1(X) =>
+    case Tuple1(Y) => // unreachable
+    case _      =>
+  }
+}
+
+
+sealed abstract class B;
+case object True extends B;
+case object False extends B;
+
+object Test2 {
+
+  val X: B = True
+  val Y: B = False
+
+  (null: Tuple1[B]) match {
+    case Tuple1(X) =>
+    case Tuple1(Y) => // no warning
+    case _      =>
+  }
+}
+
+object Test3 {
+  val X, O = true
+  def classify(neighbourhood: (Boolean, Boolean, Boolean)): String = {
+    neighbourhood match {
+      case (X, X, X) => "middle"
+      case (X, X, O) => "right"
+      case (O, X, X) => "left"
+      case _ => throw new IllegalArgumentException("Invalid")
+    }
+  }
+}
\ No newline at end of file