SI-7171 Consider prefix when assessing type finality.

`Type#isFinalType` determines if a type could have a
non-bottom subtype. This property is exploited by
the pattern matcher to flag impossible patterns.

This check was ignoring the type's prefix, and
incorrectly deemed that `T#A` in `trait T { final class A }`
was a final type. But it could have been subtyped by
`U#A` where `U` <:< `T`, or, more simply, by `T.this.A`.

Now, type finality requires that the prefix is stable.

The existing test cases in neg/patmat-type-check.scala
still correctly flag incompatiblities.

`isFinalType` is also used by some code that massages
pattern matches post specialization. That is actually
either broken or obsolete under virtpatmat, I've opened
SI-7172 to invesigate that.

It is also used by GenICode to determine whether to emit
the appropriate equality checks that are correct in the
face of boxing. It is possible that this change will force
the slow path in some rare cases, but it won't affect
correctness.

8 files changed, 69 insertions, 2 deletions

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index fb493fabd8..546df8a207 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -391,8 +391,8 @@ trait Types extends api.Types { self: SymbolTable =>
      *  This is assessed to be the case if the class is final,
      *  and all type parameters (if any) are invariant.
      */
-    def isFinalType =
-      typeSymbol.isFinal && (typeSymbol.typeParams forall symbolIsNonVariant)
+    def isFinalType: Boolean =
+      typeSymbol.isFinal && (typeSymbol.typeParams forall symbolIsNonVariant) && prefix.isStable
 
     /** Is this type completed (i.e. not a lazy type)? */
     def isComplete: Boolean = true
diff --git a/test/files/neg/t7171.check b/test/files/neg/t7171.check
new file mode 100644
index 0000000000..8bdf08129b
--- /dev/null
+++ b/test/files/neg/t7171.check
@@ -0,0 +1,7 @@
+t7171.scala:2: error: The outer reference in this type test cannot be checked at run time.
+  final case class A()
+                   ^
+t7171.scala:9: error: The outer reference in this type test cannot be checked at run time.
+    case _: A => true; case _ => false
+          ^
+two errors found
diff --git a/test/files/neg/t7171.flags b/test/files/neg/t7171.flags
new file mode 100644
index 0000000000..464cc20ea6
--- /dev/null
+++ b/test/files/neg/t7171.flags
@@ -0,0 +1 @@
+-Xfatal-warnings -unchecked
\ No newline at end of file
diff --git a/test/files/neg/t7171.scala b/test/files/neg/t7171.scala
new file mode 100644
index 0000000000..534b2070a3
--- /dev/null
+++ b/test/files/neg/t7171.scala
@@ -0,0 +1,11 @@
+trait T {
+  final case class A()
+
+  // Was:
+  // error: scrutinee is incompatible with pattern type;
+  // found   : T.this.A
+  // required: T#A
+  def foo(a: T#A) = a match {
+    case _: A => true; case _ => false
+  }
+}
diff --git a/test/files/neg/t7171b.check b/test/files/neg/t7171b.check
new file mode 100644
index 0000000000..bd6b2bcfb4
--- /dev/null
+++ b/test/files/neg/t7171b.check
@@ -0,0 +1,10 @@
+t7171b.scala:2: error: The outer reference in this type test cannot be checked at run time.
+  final case class A()
+                   ^
+t7171b.scala:8: error: The outer reference in this type test cannot be checked at run time.
+    case _: A => true; case _ => false
+          ^
+t7171b.scala:13: error: The outer reference in this type test cannot be checked at run time.
+    case _: A => true; case _ => false
+          ^
+three errors found
diff --git a/test/files/neg/t7171b.flags b/test/files/neg/t7171b.flags
new file mode 100644
index 0000000000..464cc20ea6
--- /dev/null
+++ b/test/files/neg/t7171b.flags
@@ -0,0 +1 @@
+-Xfatal-warnings -unchecked
\ No newline at end of file
diff --git a/test/files/neg/t7171b.scala b/test/files/neg/t7171b.scala
new file mode 100644
index 0000000000..53c7787f8b
--- /dev/null
+++ b/test/files/neg/t7171b.scala
@@ -0,0 +1,15 @@
+trait T {
+  final case class A()
+}
+
+final class U extends T {
+  // this match should also not be deemed impossible
+  def foo(a: U#A) = a match {
+    case _: A => true; case _ => false
+  }
+
+  // this match should also not be deemed impossible
+  def bar(a: T#A) = a match {
+    case _: A => true; case _ => false
+  }
+}
diff --git a/test/files/run/t7171.scala b/test/files/run/t7171.scala
new file mode 100644
index 0000000000..97585b9860
--- /dev/null
+++ b/test/files/run/t7171.scala
@@ -0,0 +1,22 @@
+trait T {
+  final case class A()
+
+  // Was:
+  // error: scrutinee is incompatible with pattern type;
+  // found   : T.this.A
+  // required: T#A
+  def foo(a: T#A) = a match {
+    case _: A => true; case _ => false
+  }
+}
+
+object Test extends App {
+  val t1 = new T {}
+  val t2 = new T {}
+  val a1 = new t1.A()
+  val a2 = new t1.A()
+  assert(t1.foo(a1))
+  // as noted in the unchecked warning (tested in the corresponding neg test),
+  // the outer pointer isn't checked
+  assert(t1.foo(a2))
+}