SI-7214 outer check based on dealiased pattern type.

A Typed Pattern (_: T) is more than `.isInstanceOf`: if `T` is a
path dependent type, the scrutinee's $outer reference is also
compared against the prefix of `T`.

The code that synthesises this is split into two places.
`needsOuterCheck` determines whether to add this check, based on
the type `T`, and the type of the scrutinee. If it gives the
go-ahead, `treeCondStrategy.outerCheck` synthesizes the check.

The new test case demonstrates the problems caused by the failure
to dealias in `needsOuterCheck`: it could either wrongly lead to
synthesis of an outer test (which would crash), or wrongly omit
the outer test (meaning overly liberal matching.)

A simple `dealias` remedies this. `dealiasWiden` is *not*
appropriate here; we need to keep hold of singleton types.

I'll also note that there is already a little slack between these
methods, as commented:

> ExplicitOuter replaces `Select(q, outerSym) OBJ_EQ expectedPrefix`
> by `Select(q, > outerAccessor(outerSym.owner)) OBJ_EQ expectedPrefix`
> if there's an outer accessor, otherwise the condition becomes `true`
> TODO: can we improve needsOuterTest so there's always an outerAccessor?

So this is probably a fragile area that warrants a careful review
with a view to design improvements.

1 files changed, 57 insertions, 0 deletions

diff --git a/test/files/run/t7214.scala b/test/files/run/t7214.scala
new file mode 100644
index 0000000000..ff1ea8082d
--- /dev/null
+++ b/test/files/run/t7214.scala
@@ -0,0 +1,57 @@
+// pattern matcher crashes here trying to synthesize an uneeded outer test.
+//     no-symbol does not have an owner
+// at scala.reflect.internal.SymbolTable.abort(SymbolTable.scala:49)
+// at scala.tools.nsc.Global.abort(Global.scala:253)
+// at scala.reflect.internal.Symbols$NoSymbol.owner(Symbols.scala:3248)
+// at scala.reflect.internal.Symbols$Symbol.effectiveOwner(Symbols.scala:678)
+// at scala.reflect.internal.Symbols$Symbol.isDefinedInPackage(Symbols.scala:664)
+// at scala.reflect.internal.TreeGen.mkAttributedSelect(TreeGen.scala:188)
+// at scala.reflect.internal.TreeGen.mkAttributedRef(TreeGen.scala:124)
+// at scala.tools.nsc.ast.TreeDSL$CODE$.REF(TreeDSL.scala:308)
+// at scala.tools.nsc.typechecker.PatternMatching$TreeMakers$TypeTestTreeMaker$treeCondStrategy$.outerTest(PatternMatching.scala:1209)
+class Crash {
+  type Alias = C#T
+
+  val c = new C
+  val t = new c.T
+
+  // Crash via a Typed Pattern...
+  (t: Any) match {
+    case e: Alias =>
+  }
+
+  // ... or via a Typed Extractor Pattern.
+  object Extractor {
+    def unapply(a: Alias): Option[Any] = None
+  }
+  (t: Any) match {
+    case Extractor() =>
+    case _ =>
+  }
+
+  // checking that correct outer tests are applied when
+  // aliases for path dependent types are involved.
+  val c2 = new C
+  type CdotT = c.T
+  type C2dotT = c2.T
+
+  val outerField = t.getClass.getDeclaredFields.find(_.getName contains ("outer")).get
+  outerField.setAccessible(true)
+
+  (t: Any) match {
+    case _: C2dotT =>
+      println(s"!!! wrong match. t.outer=${outerField.get(t)} / c2 = $c2") // this matches on 2.10.0
+    case _: CdotT =>
+    case _ =>
+      println(s"!!! wrong match. t.outer=${outerField.get(t)} / c = $c")
+  }
+}
+
+class C {
+  class T
+}
+
+object Test extends App {
+  new Crash
+}
+