From 696dcdfcdb40ee3dbd2ba63f8281b87cde787a00 Mon Sep 17 00:00:00 2001
From: Jason Zaugg <jzaugg@gmail.com>
Date: Mon, 25 Feb 2013 17:58:11 +0100
Subject: [PATCH 1/2] SI-7126 Account for the alias types that don't dealias.

After this change:

    qbin/scalac -Ydebug test/files/pos/t7126.scala 2>&1 | grep warning
    warning: dropExistential did not progress dealiasing Test.this.T[Test.this.T], see SI-7126
    one warning found

T[T]? Really? The true bug lies somewhere else; the comments of
the ticket illuminate the general areas of concern.
---
 src/compiler/scala/tools/nsc/typechecker/Typers.scala |  9 +++++++--
 test/files/pos/t7126.scala                            | 11 +++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)
 create mode 100644 test/files/pos/t7126.scala

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index cff3f4f0fa18..4fb68fc2a9cb 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -222,8 +222,13 @@ trait Typers extends Modes with Adaptations with Tags {
         new SubstWildcardMap(tparams).apply(tp)
       case TypeRef(_, sym, _) if sym.isAliasType =>
         val tp0 = tp.dealias
-        val tp1 = dropExistential(tp0)
-        if (tp1 eq tp0) tp else tp1
+        if (tp eq tp0) {
+          debugwarn(s"dropExistential did not progress dealiasing $tp, see SI-7126")
+          tp
+        } else {
+          val tp1 = dropExistential(tp0)
+          if (tp1 eq tp0) tp else tp1
+        }
       case _ => tp
     }
 
diff --git a/test/files/pos/t7126.scala b/test/files/pos/t7126.scala
new file mode 100644
index 000000000000..6720511e08a7
--- /dev/null
+++ b/test/files/pos/t7126.scala
@@ -0,0 +1,11 @@
+import language._
+
+object Test {
+  type T = Any
+  boom(???): Option[T] // SOE
+  def boom[CC[U]](t : CC[T]): Option[CC[T]] = None
+
+  // okay
+  foo(???): Option[Any]
+  def foo[CC[U]](t : CC[Any]): Option[CC[Any]] = None
+}
\ No newline at end of file

From 204b2b45a83cf057b6ece7a25ea4f15ec38b4a3f Mon Sep 17 00:00:00 2001
From: Jason Zaugg <jzaugg@gmail.com>
Date: Tue, 26 Feb 2013 17:11:43 +0100
Subject: [PATCH 2/2] SI-7126 Eliminate a source of malformed types.

The kind-polymorphic nature of Nothing and Any in
concert with type argument inference could lead to
types like `T[T]` (where `type T=Any`).

Compensatory action is taken later on to recover;
see the usages of `TypeRef#typeParamsMatchArgs`.
But this these types have a nasty property, they
can dealias to themselves. Callers recursing through
types who fail to account for this hit an infinite
recursion, as was reported in SI-7126.

This commit simply dealiases `T` when registering
the type bound in `unifySimple`.

We should try to weed out additional sources of
these types.
---
 src/reflect/scala/reflect/internal/Types.scala | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index fb493fabd86b..7299b439f83b 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -3183,12 +3183,19 @@ trait Types extends api.Types { self: SymbolTable =>
        *    ?TC[?T] <: Any
        *  }}}
        */
-      def unifySimple = (
-        (params.isEmpty || tp.typeSymbol == NothingClass || tp.typeSymbol == AnyClass) && {
+      def unifySimple = {
+        val sym = tp.typeSymbol
+        if (sym == NothingClass || sym == AnyClass) { // kind-polymorphic
+          // SI-7126 if we register some type alias `T=Any`, we can later end
+          // with malformed types like `T[T]` during type inference in
+          // `handlePolymorphicCall`. No such problem if we register `Any`.
+          addBound(sym.tpe)
+          true
+        } else if (params.isEmpty) {
           addBound(tp)
           true
-        }
-      )
+        } else false
+      }
 
       /** Full case: involving a check of the form
        *  {{{