From e872aeec6d725c38a0f8f6433d165c2cba7f5dce Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Mon, 19 Feb 2018 17:25:31 +0100
Subject: [PATCH 1/9] Fix #3989: Check that members of concrete classes are
 type-correct

This fixes the second half of #3989. Some tests had to be updated or
re-classified because they were unsound before.
---
 .../dotty/tools/dotc/typer/RefChecks.scala    | 49 +++++++++++++++++++
 tests/neg/i1240b.scala                        |  2 +-
 tests/neg/i3989.scala                         |  7 +++
 tests/{pos => neg}/t4731.scala                |  6 ++-
 .../CollectionStrawMan6.scala                 |  4 +-
 .../run/colltest6/CollectionStrawMan6_1.scala |  4 +-
 6 files changed, 65 insertions(+), 7 deletions(-)
 create mode 100644 tests/neg/i3989.scala
 rename tests/{pos => neg}/t4731.scala (51%)

diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index 2a0bafe31a2d..aeb3bd81f65c 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -594,12 +594,61 @@ object RefChecks {
           checkNoAbstractDecls(bc.asClass.superClass)
       }
 
+      // Check that every term member of this concrete class has a symbol that matches the member's type
+      // Member types are computed by intersecting the types of all members that have the same name
+      // and signature. But a member selection will pick one particular implementation, according to
+      // the rules of overriding and linearization. This method checks that the implementation has indeed
+      // a type that subsumes the full member type.
+      def checkMemberTypesOK() = {
+
+        // First compute all member names we need to check in `membersToCheck`.
+        // We do not check
+        //  - types
+        //  - synthetic members or bridges
+        //  - members in other concrete classes, since these have been checked before
+        //    (this is done for efficiency)
+        //  - members in a prefix of inherited parents that all come from Java or Scala2
+        //    (this is done to avoid false negatives since Scala2's rules for checking are different)
+        val membersToCheck = mutable.Set[Name]()
+        val seenClasses = mutable.Set[Symbol]()
+        def addDecls(cls: Symbol): Unit =
+          if (!seenClasses.contains(cls)) {
+            seenClasses.+=(cls)
+            for (mbr <- cls.info.decls)
+              if (mbr.isTerm && !mbr.is(Synthetic | Bridge) && mbr.memberCanMatchInheritedSymbols &&
+                  !membersToCheck.contains(mbr.name))
+                membersToCheck.+=(mbr.name)
+            cls.info.parents.map(_.classSymbol)
+              .filter(_.is(AbstractOrTrait))
+              .dropWhile(_.is(JavaDefined | Scala2x))
+              .foreach(addDecls)
+          }
+        addDecls(clazz)
+
+        // For each member, check that the type of its symbol, as seen from `self`
+        // can override the info of this member
+        for (name <- membersToCheck) {
+          for (mbrd <- self.member(name).alternatives) {
+            val mbr = mbrd.symbol
+            val mbrType = mbr.info.asSeenFrom(self, mbr.owner)
+            if (!mbrType.overrides(mbrd.info, matchLoosely = true))
+              ctx.errorOrMigrationWarning(
+                em"""${mbr.showLocated} is not a legal implementation of `$name' in $clazz
+                    |  its type             $mbrType
+                    |  does not conform to  ${mbrd.info}""",
+                (if (mbr.owner == clazz) mbr else clazz).pos)
+          }
+        }
+      }
+
       checkNoAbstractMembers()
       if (abstractErrors.isEmpty)
         checkNoAbstractDecls(clazz)
 
       if (abstractErrors.nonEmpty)
         ctx.error(abstractErrorMessage, clazz.pos)
+
+      checkMemberTypesOK()
     } else if (clazz.is(Trait) && !(clazz derivesFrom defn.AnyValClass)) {
       // For non-AnyVal classes, prevent abstract methods in interfaces that override
       // final members in Object; see #4431
diff --git a/tests/neg/i1240b.scala b/tests/neg/i1240b.scala
index 2d23db61470b..8f2ec81d4702 100644
--- a/tests/neg/i1240b.scala
+++ b/tests/neg/i1240b.scala
@@ -9,4 +9,4 @@ abstract class A[X] extends T[X] {
 trait U[X] extends T[X] {
   abstract override def foo(x: X): X = super.foo(x)
 }
-object Test extends A[String] with U[String] // error: accidental override
+object Test extends A[String] with U[String] // error: accidental override // error: merge error
diff --git a/tests/neg/i3989.scala b/tests/neg/i3989.scala
new file mode 100644
index 000000000000..6cdcad568561
--- /dev/null
+++ b/tests/neg/i3989.scala
@@ -0,0 +1,7 @@
+object Test extends App {
+  trait A[+X] { def get: X }
+  case class B[X](x: X) extends A[X] { def get: X = x }
+  class C[X](x: Any) extends B[Any](x) with A[X] // error: not a legal implementation of `get'
+  def g(a: A[Int]): Int = a.get
+  g(new C[Int]("foo"))
+}
\ No newline at end of file
diff --git a/tests/pos/t4731.scala b/tests/neg/t4731.scala
similarity index 51%
rename from tests/pos/t4731.scala
rename to tests/neg/t4731.scala
index d457543c1f4c..99bea6a8fde4 100644
--- a/tests/pos/t4731.scala
+++ b/tests/neg/t4731.scala
@@ -1,14 +1,16 @@
 import java.util.Comparator
 
-trait Trait1[T] { def foo(arg: Comparator[T]): Unit }
+trait Trait1[T] { def foo(arg: Comparator[T]): String }
 
 trait Trait2[T] extends Trait1[T] { def foo(arg: Comparator[String]): Int = 0 }
 
-class Class1 extends Trait2[String] { }
+class Class1 extends Trait2[String] { } // error: not a legal implementation of `foo'
 
 object Test {
   def main(args: Array[String]): Unit = {
     val c = new Class1
     c.foo(Ordering[String])
+    val t: Trait1[String] = c
+    val x: String = t.foo(Ordering[String])
   }
 }
diff --git a/tests/pos-special/strawman-collections/CollectionStrawMan6.scala b/tests/pos-special/strawman-collections/CollectionStrawMan6.scala
index c812a85f442a..14b2bbae0d1b 100644
--- a/tests/pos-special/strawman-collections/CollectionStrawMan6.scala
+++ b/tests/pos-special/strawman-collections/CollectionStrawMan6.scala
@@ -415,7 +415,7 @@ object CollectionStrawMan6 extends LowPriority {
   /** Concrete collection type: List */
   sealed trait List[+A]
   extends LinearSeq[A]
-     with SeqLike[A, List]
+     with LinearSeqLike[A, List]
      with Buildable[A, List[A]] {
 
     def fromIterable[B](c: Iterable[B]): List[B] = List.fromIterable(c)
@@ -604,7 +604,7 @@ object CollectionStrawMan6 extends LowPriority {
   }
 
   class LazyList[+A](expr: => LazyList.Evaluated[A])
-  extends LinearSeq[A] with SeqLike[A, LazyList] {
+  extends LinearSeq[A] with LinearSeqLike[A, LazyList] {
     private[this] var evaluated = false
     private[this] var result: LazyList.Evaluated[A] = _
 
diff --git a/tests/run/colltest6/CollectionStrawMan6_1.scala b/tests/run/colltest6/CollectionStrawMan6_1.scala
index 8f5c38104dae..17c28d74ef0f 100644
--- a/tests/run/colltest6/CollectionStrawMan6_1.scala
+++ b/tests/run/colltest6/CollectionStrawMan6_1.scala
@@ -416,7 +416,7 @@ object CollectionStrawMan6 extends LowPriority {
   /** Concrete collection type: List */
   sealed trait List[+A]
   extends LinearSeq[A]
-     with SeqLike[A, List]
+     with LinearSeqLike[A, List]
      with Buildable[A, List[A]] {
 
     def fromIterable[B](c: Iterable[B]): List[B] = List.fromIterable(c)
@@ -605,7 +605,7 @@ object CollectionStrawMan6 extends LowPriority {
   }
 
   class LazyList[+A](expr: => LazyList.Evaluated[A])
-  extends LinearSeq[A] with SeqLike[A, LazyList] {
+  extends LinearSeq[A] with LinearSeqLike[A, LazyList] {
     private[this] var evaluated = false
     private[this] var result: LazyList.Evaluated[A] = _
 

From faae4da6385f5014ce68e5ca7c1e02029c4863da Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Mon, 19 Feb 2018 18:00:38 +0100
Subject: [PATCH 2/9] Use util.HashSet for member checking

It's more efficient. Let's see whether that makes a difference.
---
 compiler/src/dotty/tools/dotc/typer/RefChecks.scala | 10 +++++-----
 compiler/src/dotty/tools/dotc/util/HashSet.scala    |  1 +
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index aeb3bd81f65c..bf4f7ce75d79 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -608,16 +608,16 @@ object RefChecks {
         //  - members in other concrete classes, since these have been checked before
         //    (this is done for efficiency)
         //  - members in a prefix of inherited parents that all come from Java or Scala2
-        //    (this is done to avoid false negatives since Scala2's rules for checking are different)
-        val membersToCheck = mutable.Set[Name]()
-        val seenClasses = mutable.Set[Symbol]()
+        //    (this is done to avoid false positives since Scala2's rules for checking are different)
+        val membersToCheck = new util.HashSet[Name](4096)
+        val seenClasses = new util.HashSet[Symbol](256)
         def addDecls(cls: Symbol): Unit =
           if (!seenClasses.contains(cls)) {
-            seenClasses.+=(cls)
+            seenClasses.addEntry(cls)
             for (mbr <- cls.info.decls)
               if (mbr.isTerm && !mbr.is(Synthetic | Bridge) && mbr.memberCanMatchInheritedSymbols &&
                   !membersToCheck.contains(mbr.name))
-                membersToCheck.+=(mbr.name)
+                membersToCheck.addEntry(mbr.name)
             cls.info.parents.map(_.classSymbol)
               .filter(_.is(AbstractOrTrait))
               .dropWhile(_.is(JavaDefined | Scala2x))
diff --git a/compiler/src/dotty/tools/dotc/util/HashSet.scala b/compiler/src/dotty/tools/dotc/util/HashSet.scala
index 9ffdeac132de..3dca9efadc4e 100644
--- a/compiler/src/dotty/tools/dotc/util/HashSet.scala
+++ b/compiler/src/dotty/tools/dotc/util/HashSet.scala
@@ -7,6 +7,7 @@ class HashSet[T >: Null <: AnyRef](powerOfTwoInitialCapacity: Int, loadFactor: F
   private[this] var limit: Int = _
   private[this] var table: Array[AnyRef] = _
 
+  assert(Integer.bitCount(powerOfTwoInitialCapacity) == 1)
   protected def isEqual(x: T, y: T): Boolean = x.equals(y)
 
   // Counters for Stats

From 08cef8ca20cab461b81c78f17b64417588b8fb6b Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 20 Feb 2018 13:09:20 +0100
Subject: [PATCH 3/9] Disable strawman tests

Plugging the soundness hole in #3989 unveiled problems in the strawman. These need to be
fixed before we can test it again in dotty.
---
 compiler/test/dotty/tools/dotc/IdempotencyTests.scala | 6 ++++--
 compiler/test/dotty/tools/dotc/LinkTests.scala        | 4 +++-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/compiler/test/dotty/tools/dotc/IdempotencyTests.scala b/compiler/test/dotty/tools/dotc/IdempotencyTests.scala
index 2e7d8e8c58b9..8c969d6ff011 100644
--- a/compiler/test/dotty/tools/dotc/IdempotencyTests.scala
+++ b/compiler/test/dotty/tools/dotc/IdempotencyTests.scala
@@ -64,12 +64,14 @@ class IdempotencyTests extends ParallelTesting {
     }
     val allChecks = {
       check("CheckOrderIdempotency") +
-      check("CheckStrawmanIdempotency") +
+      // Disabled until strawman is fixed
+      // check("CheckStrawmanIdempotency") +
       check("CheckPosIdempotency")
     }
 
     val allTests = {
-      strawmanIdempotency +
+      // Disabled until strawman is fixed
+      // strawmanIdempotency +
       orderIdempotency +
       posIdempotency
     }
diff --git a/compiler/test/dotty/tools/dotc/LinkTests.scala b/compiler/test/dotty/tools/dotc/LinkTests.scala
index 6c2ed2628ea3..29a81aed3593 100644
--- a/compiler/test/dotty/tools/dotc/LinkTests.scala
+++ b/compiler/test/dotty/tools/dotc/LinkTests.scala
@@ -26,7 +26,9 @@ class LinkTests extends ParallelTesting {
   def testFilter = Properties.testsFilter
 
 
-  @Test def linkTest: Unit = {
+  // Disabled until strawman is fixed
+  // @Test
+  def linkTest: Unit = {
     // Setup and compile libraries
     val strawmanLibGroup = TestGroup("linkTest/strawmanLibrary")
     val strawmanLibTestGroup = TestGroup(strawmanLibGroup + "/tests")

From 07ced9d9f2bb304ea40f254a8ac40768f08502e5 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 20 Feb 2018 13:50:09 +0100
Subject: [PATCH 4/9] Be more careful when constraining parameter types wrt
 scrutinees

i3989a.scala gives an example which is rejected under the new scheme.
This would pass and fail at runtime under the previous scheme.
---
 .../dotty/tools/dotc/typer/Inferencing.scala  | 53 +++++++++++++++++++
 .../src/dotty/tools/dotc/typer/Typer.scala    |  2 +-
 tests/neg/i3989a.scala                        | 10 ++++
 tests/pos/t3880.scala                         |  5 +-
 tests/pos/t6084.scala                         |  5 +-
 5 files changed, 71 insertions(+), 4 deletions(-)
 create mode 100644 tests/neg/i3989a.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
index bc90ec7df8ae..fcc75ace61d1 100644
--- a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
@@ -154,6 +154,59 @@ object Inferencing {
       tree
   }
 
+  /** Derive information about a pattern type by comparing it with some variant of the
+   *  static scrutinee type. We have the following situation in case of a (dynamic) pattern match:
+   *
+   *       StaticScrutineeType           PatternType
+   *                         \            /
+   *                      DynamicScrutineeType
+   *
+   *  If `PatternType` is not a subtype of `StaticScrutineeType, there's no information to be gained.
+   *  Now let's say we can prove that `PatternType <: StaticScrutineeType`.
+   *
+   *            StaticScrutineeType
+   *                  |         \
+   *                  |          \
+   *                  |           \
+   *                  |            PatternType
+   *                  |          /
+   *               DynamicScrutineeType
+   *
+   *  What can we say about the relationship of parameter types between `PatternType` and
+   *  `DynamicScrutineeType`?
+   *
+   *   - If `DynamicScrutineeType` refines the type parameters of `StaticScrutineeType`
+   *     in the same way as `PatternType`, the subtype test `PatternType <:< StaticScrutineeType`
+   *     tells us all we need to know.
+   *   - Otherwise, if variant refinement is a possibility we can only make predictions
+   *     about invariant parameters of `StaticScrutineeType`. Hence we do a subtype test
+   *     where `PatternType <: widenVariantParams(StaticScrutineeType)`, where `widenVariantParams`
+   *     replaces all type argument of variant parameters with empty bounds.
+   */
+  def constrainPatternType(tp: Type, pt: Type)(implicit ctx: Context) = {
+    def refinementIsInvariant(tp: Type): Boolean = tp match {
+      case tp: ClassInfo => tp.cls.is(Final) || tp.cls.is(Case)
+      case tp: TypeProxy => refinementIsInvariant(tp.underlying)
+      case tp: AndOrType => refinementIsInvariant(tp.tp1) && refinementIsInvariant(tp.tp2)
+      case _ => false
+    }
+
+    def widenVariantParams = new TypeMap {
+      def apply(tp: Type) = mapOver(tp) match {
+        case tp @ AppliedType(tycon, args) =>
+          val args1 = args.zipWithConserve(tycon.typeParams)((arg, tparam) =>
+            if (tparam.paramVariance != 0) TypeBounds.empty else arg
+          )
+          tp.derivedAppliedType(tycon, args1)
+        case tp =>
+          tp
+      }
+    }
+
+    val widePt = if (ctx.scala2Mode || refinementIsInvariant(tp)) pt else widenVariantParams(pt)
+    (tp <:< widePt)(ctx.addMode(Mode.GADTflexible))
+  }
+
   /** The list of uninstantiated type variables bound by some prefix of type `T` which
    *  occur in at least one formal parameter type of a prefix application.
    *  Considered prefixes are:
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 438b3ef9b279..4b9ce6bc57a2 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -569,7 +569,7 @@ class Typer extends Namer
       def typedTpt = checkSimpleKinded(typedType(tree.tpt))
       def handlePattern: Tree = {
         val tpt1 = typedTpt
-        if (!ctx.isAfterTyper) tpt1.tpe.<:<(pt)(ctx.addMode(Mode.GADTflexible))
+        if (!ctx.isAfterTyper) constrainPatternType(tpt1.tpe, pt)
         // special case for an abstract type that comes with a class tag
         tryWithClassTag(ascription(tpt1, isWildcard = true), pt)
       }
diff --git a/tests/neg/i3989a.scala b/tests/neg/i3989a.scala
new file mode 100644
index 000000000000..8ff02ac0c2f1
--- /dev/null
+++ b/tests/neg/i3989a.scala
@@ -0,0 +1,10 @@
+object Test extends App {
+  trait A[+X]
+  class B[+X](val x: X) extends A[X]
+  class C[+X](x: Any) extends B[Any](x) with A[X]
+  def f(a: A[Int]): Int = a match {
+    case a: B[_] => a.x // error
+    case _ => 0
+  }
+  f(new C[Int]("foo"))
+}
diff --git a/tests/pos/t3880.scala b/tests/pos/t3880.scala
index f778eb71a81a..d7b567e1901a 100644
--- a/tests/pos/t3880.scala
+++ b/tests/pos/t3880.scala
@@ -1,11 +1,12 @@
 abstract class Bar[+B] {
 }
-abstract class C1[+B] extends Bar[B] {
+final class C1[+B] extends Bar[B] {
   private[this] def g(x: C1[B]): Unit = ()
 
   // this method is fine: notice that it allows the call to g,
   // which requires C1[B], even though we matched on C1[_].
-  // (That is good news.)
+  // (That is good news, but is sound only because C1 is final; see #3989
+  // and compare with i3989a.scala.
   private[this] def f1(x: Bar[B]): Unit = x match {
     case x: C1[_] => g(x)
   }
diff --git a/tests/pos/t6084.scala b/tests/pos/t6084.scala
index 1aa1fed391e8..1107d9a03885 100644
--- a/tests/pos/t6084.scala
+++ b/tests/pos/t6084.scala
@@ -1,11 +1,14 @@
 package object foo { type X[T, U] = (T => U) }
 
 package foo {
-  abstract class Foo[T, U](val d: T => U) extends (T => U) {
+  // Note that Foo must be final because of #3989.
+  final class Foo[T, U](val d: T => U) extends (T => U) {
     def f1(r: X[T, U])           = r match { case x: Foo[_,_] => x.d }  // inferred ok
     def f2(r: X[T, U]): (T => U) = r match { case x: Foo[_,_] => x.d }  // dealiased ok
     def f3(r: X[T, U]): X[T, U]  = r match { case x: Foo[_,_] => x.d }  // alias not ok
 
+    def apply(x: T): U = d(x)
+
     // x.d : foo.this.package.type.X[?scala.reflect.internal.Types$NoPrefix$?.T, ?scala.reflect.internal.Types$NoPrefix$?.U] ~>scala.this.Function1[?scala.reflect.internal.Types$NoPrefix$?.T, ?scala.reflect.internal.Types$NoPrefix$?.U]
     //  at scala.Predef$.assert(Predef.scala:170)
     //  at scala.tools.nsc.Global.assert(Global.scala:235)

From 7bac0a2e40bc53be48719247e3899a4e09cb7825 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 20 Feb 2018 14:38:48 +0100
Subject: [PATCH 5/9] Ensure invariant refinement for classes extended case
 classes

---
 .../dotty/tools/dotc/typer/Inferencing.scala  |  7 ++++--
 .../dotty/tools/dotc/typer/RefChecks.scala    | 22 +++++++++++++++++++
 tests/neg/i3989b.scala                        | 10 +++++++++
 3 files changed, 37 insertions(+), 2 deletions(-)
 create mode 100644 tests/neg/i3989b.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
index fcc75ace61d1..49cb38cbb0a9 100644
--- a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
@@ -176,12 +176,15 @@ object Inferencing {
    *  `DynamicScrutineeType`?
    *
    *   - If `DynamicScrutineeType` refines the type parameters of `StaticScrutineeType`
-   *     in the same way as `PatternType`, the subtype test `PatternType <:< StaticScrutineeType`
-   *     tells us all we need to know.
+   *     in the same way as `PatternType` ("invariant refinement"), the subtype test
+   *     `PatternType <:< StaticScrutineeType` tells us all we need to know.
    *   - Otherwise, if variant refinement is a possibility we can only make predictions
    *     about invariant parameters of `StaticScrutineeType`. Hence we do a subtype test
    *     where `PatternType <: widenVariantParams(StaticScrutineeType)`, where `widenVariantParams`
    *     replaces all type argument of variant parameters with empty bounds.
+   *
+   *  Invariant refinement can be assumed if `PatternType`'s class(es) are final or
+   *  case classes (because of `RefChecks#checkCaseClassInheritanceInvariant`).
    */
   def constrainPatternType(tp: Type, pt: Type)(implicit ctx: Context) = {
     def refinementIsInvariant(tp: Type): Boolean = tp match {
diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index bf4f7ce75d79..1f1b12d8e211 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -641,6 +641,27 @@ object RefChecks {
         }
       }
 
+      /** Check that inheriting a case class does not constitute a variant refinement
+       *  of a base type of the case class. It is because of this restriction that we
+       *  can assume invariant refinement for case classes in `constrainPatternType`.
+       */
+      def checkCaseClassInheritanceInvariant() = {
+        for (caseCls <- clazz.info.baseClasses.tail.find(_.is(Case)))
+          for (bc <- caseCls.info.baseClasses.tail)
+            if (bc.typeParams.exists(_.paramVariance != 0)) {
+              val caseBT = self.baseType(caseCls)
+              val thisBT = self.baseType(bc)
+              val combinedBT = caseBT.baseType(bc)
+              if (!(thisBT =:= combinedBT))
+                ctx.errorOrMigrationWarning(
+                  em"""illegal inheritance: $clazz inherits case $caseCls
+                      |but the two have different base type instances for $bc.
+                      |
+                      |  Basetype for $clazz: $thisBT
+                      |  Basetype via $caseCls: $combinedBT""", clazz.pos)
+            }
+      }
+
       checkNoAbstractMembers()
       if (abstractErrors.isEmpty)
         checkNoAbstractDecls(clazz)
@@ -649,6 +670,7 @@ object RefChecks {
         ctx.error(abstractErrorMessage, clazz.pos)
 
       checkMemberTypesOK()
+      checkCaseClassInheritanceInvariant()
     } else if (clazz.is(Trait) && !(clazz derivesFrom defn.AnyValClass)) {
       // For non-AnyVal classes, prevent abstract methods in interfaces that override
       // final members in Object; see #4431
diff --git a/tests/neg/i3989b.scala b/tests/neg/i3989b.scala
new file mode 100644
index 000000000000..82935fab6daa
--- /dev/null
+++ b/tests/neg/i3989b.scala
@@ -0,0 +1,10 @@
+object Test extends App {
+  trait A[+X]
+  case class B[+X](val x: X) extends A[X]
+  class C[+X](x: Any) extends B[Any](x) with A[X] // error
+  def f(a: A[Int]): Int = a match {
+    case a: B[_] => a.x
+    case _ => 0
+  }
+  f(new C[Int]("foo"))
+}

From f00ac9549ddd891403461e7e8c7ee7535eb3babc Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 20 Feb 2018 16:37:45 +0100
Subject: [PATCH 6/9] Use constrainPatternType also for unapply patterns

---
 .../src/dotty/tools/dotc/typer/Applications.scala |  2 +-
 .../src/dotty/tools/dotc/typer/Inferencing.scala  |  4 ++--
 compiler/src/dotty/tools/dotc/typer/Typer.scala   |  2 +-
 tests/neg/i3989b.scala                            |  2 +-
 tests/neg/i3989c.scala                            | 15 +++++++++++++++
 5 files changed, 20 insertions(+), 5 deletions(-)
 create mode 100644 tests/neg/i3989c.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Applications.scala b/compiler/src/dotty/tools/dotc/typer/Applications.scala
index fc2d4eae6422..11a77ec449ad 100644
--- a/compiler/src/dotty/tools/dotc/typer/Applications.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Applications.scala
@@ -933,7 +933,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
      *   - If a type proxy P is not a reference to a class, P's supertype is in G
      */
     def isSubTypeOfParent(subtp: Type, tp: Type)(implicit ctx: Context): Boolean =
-      if (subtp <:< tp) true
+      if (constrainPatternType(subtp, tp)) true
       else tp match {
         case tp: TypeRef if tp.symbol.isClass => isSubTypeOfParent(subtp, tp.firstParent)
         case tp: TypeProxy => isSubTypeOfParent(subtp, tp.superType)
diff --git a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
index 49cb38cbb0a9..c5caf8d8a0ba 100644
--- a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
@@ -186,7 +186,7 @@ object Inferencing {
    *  Invariant refinement can be assumed if `PatternType`'s class(es) are final or
    *  case classes (because of `RefChecks#checkCaseClassInheritanceInvariant`).
    */
-  def constrainPatternType(tp: Type, pt: Type)(implicit ctx: Context) = {
+  def constrainPatternType(tp: Type, pt: Type)(implicit ctx: Context): Boolean = {
     def refinementIsInvariant(tp: Type): Boolean = tp match {
       case tp: ClassInfo => tp.cls.is(Final) || tp.cls.is(Case)
       case tp: TypeProxy => refinementIsInvariant(tp.underlying)
@@ -207,7 +207,7 @@ object Inferencing {
     }
 
     val widePt = if (ctx.scala2Mode || refinementIsInvariant(tp)) pt else widenVariantParams(pt)
-    (tp <:< widePt)(ctx.addMode(Mode.GADTflexible))
+    tp <:< widePt
   }
 
   /** The list of uninstantiated type variables bound by some prefix of type `T` which
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 4b9ce6bc57a2..f4ae59e887cb 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -569,7 +569,7 @@ class Typer extends Namer
       def typedTpt = checkSimpleKinded(typedType(tree.tpt))
       def handlePattern: Tree = {
         val tpt1 = typedTpt
-        if (!ctx.isAfterTyper) constrainPatternType(tpt1.tpe, pt)
+        if (!ctx.isAfterTyper) constrainPatternType(tpt1.tpe, pt)(ctx.addMode(Mode.GADTflexible))
         // special case for an abstract type that comes with a class tag
         tryWithClassTag(ascription(tpt1, isWildcard = true), pt)
       }
diff --git a/tests/neg/i3989b.scala b/tests/neg/i3989b.scala
index 82935fab6daa..fc44f40f8a81 100644
--- a/tests/neg/i3989b.scala
+++ b/tests/neg/i3989b.scala
@@ -3,7 +3,7 @@ object Test extends App {
   case class B[+X](val x: X) extends A[X]
   class C[+X](x: Any) extends B[Any](x) with A[X] // error
   def f(a: A[Int]): Int = a match {
-    case a: B[_] => a.x
+    case B(i) => i
     case _ => 0
   }
   f(new C[Int]("foo"))
diff --git a/tests/neg/i3989c.scala b/tests/neg/i3989c.scala
new file mode 100644
index 000000000000..1d631ea1273d
--- /dev/null
+++ b/tests/neg/i3989c.scala
@@ -0,0 +1,15 @@
+import scala.Option
+object Test extends App {
+  trait A[+X]
+  class B[+X](val x: X) extends A[X]
+  object B {
+    def unapply[X](b: B[X]): Option[X] = Some(b.x)
+  }
+
+  class C[+X](x: Any) extends B[Any](x) with A[X]
+  def f(a: A[Int]): Int = a match {
+    case B(i) => i   // error
+    case _ => 0
+  }
+  f(new C[Int]("foo"))
+}

From f89d2073ce8af77c6e6e3b1cbe49d486457aae96 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 20 Feb 2018 21:51:42 +0100
Subject: [PATCH 7/9] Close the protected[local] escape hatch

---
 .../tools/dotc/typer/VarianceChecker.scala    | 12 ++++++----
 tests/neg/i3989d.scala                        | 18 +++++++++++++++
 tests/neg/t3272.scala                         |  8 +++++++
 tests/{pos => neg}/t7093.scala                | 22 +++++++++----------
 .../CollectionStrawMan5.scala                 |  8 +++----
 .../CollectionStrawMan6.scala                 |  8 +++----
 tests/pos/GenericTraversableTemplate.scala    |  2 +-
 tests/pos/t3272.scala                         |  8 -------
 tests/pos/t4345.scala                         |  7 ------
 tests/pos/variances-local.scala               |  7 ------
 tests/run/CollectionTests.scala               |  6 ++---
 .../run/colltest5/CollectionStrawMan5_1.scala |  8 +++----
 .../run/colltest6/CollectionStrawMan6_1.scala |  8 +++----
 13 files changed, 64 insertions(+), 58 deletions(-)
 create mode 100644 tests/neg/i3989d.scala
 create mode 100644 tests/neg/t3272.scala
 rename tests/{pos => neg}/t7093.scala (62%)
 delete mode 100644 tests/pos/t3272.scala
 delete mode 100644 tests/pos/t4345.scala
 delete mode 100644 tests/pos/variances-local.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala b/compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala
index 39d635863426..0ba36c2ac306 100644
--- a/compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala
+++ b/compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala
@@ -32,7 +32,7 @@ class VarianceChecker()(implicit ctx: Context) {
     def ignoreVarianceIn(base: Symbol): Boolean = (
          base.isTerm
       || base.is(Package)
-      || base.is(Local)
+      || base.is(PrivateLocal)
     )
 
     /** The variance of a symbol occurrence of `tvar` seen at the level of the definition of `base`.
@@ -112,9 +112,13 @@ class VarianceChecker()(implicit ctx: Context) {
     def checkVariance(sym: Symbol, pos: Position) = Validator.validateDefinition(sym) match {
       case Some(VarianceError(tvar, required)) =>
         def msg = i"${varianceString(tvar.flags)} $tvar occurs in ${varianceString(required)} position in type ${sym.info} of $sym"
-        if (ctx.scala2Mode && sym.owner.isConstructor) {
+        if (ctx.scala2Mode &&
+            (sym.owner.isConstructor || sym.ownersIterator.exists(_.is(ProtectedLocal)))) {
           ctx.migrationWarning(s"According to new variance rules, this is no longer accepted; need to annotate with @uncheckedVariance:\n$msg", pos)
-          patch(Position(pos.end), " @scala.annotation.unchecked.uncheckedVariance") // TODO use an import or shorten if possible
+            // patch(Position(pos.end), " @scala.annotation.unchecked.uncheckedVariance")
+            // Patch is disabled until two TODOs are solved:
+            // TODO use an import or shorten if possible
+            // TODO need to use a `:' if annotation is on term
         }
         else ctx.error(msg, pos)
       case None =>
@@ -125,7 +129,7 @@ class VarianceChecker()(implicit ctx: Context) {
       // No variance check for private/protected[this] methods/values.
       def skip =
         !sym.exists ||
-        sym.is(Local) || // !!! watch out for protected local!
+        sym.is(PrivateLocal) ||
         sym.is(TypeParam) && sym.owner.isClass // already taken care of in primary constructor of class
       tree match {
         case defn: MemberDef if skip =>
diff --git a/tests/neg/i3989d.scala b/tests/neg/i3989d.scala
new file mode 100644
index 000000000000..e9260bc900ff
--- /dev/null
+++ b/tests/neg/i3989d.scala
@@ -0,0 +1,18 @@
+trait A[+_X] {
+  protected[this] type X = _X // error: variance
+  def f: X
+}
+
+trait B extends A[B] {
+  def f: X = new B {}
+}
+
+class C extends B with A[C] {
+  // should be required because the inherited f is of type B, not C
+  // override def f: X = new C
+}
+
+object Test extends App {
+  val c1 = new C
+  val c2: C = c1.f
+}
diff --git a/tests/neg/t3272.scala b/tests/neg/t3272.scala
new file mode 100644
index 000000000000..b1c90cae5f90
--- /dev/null
+++ b/tests/neg/t3272.scala
@@ -0,0 +1,8 @@
+trait A {
+  trait C[+T] {
+    protected[this] def f(t: T): Unit = {} // error: covariant type T occurs in contravariant position in type T of value t
+  }
+  trait D[T] extends C[T] {
+    def g(t: T): Unit = { f(t) }
+  }
+}
diff --git a/tests/pos/t7093.scala b/tests/neg/t7093.scala
similarity index 62%
rename from tests/pos/t7093.scala
rename to tests/neg/t7093.scala
index 287b7a78c726..dc5c25ac4f2f 100644
--- a/tests/pos/t7093.scala
+++ b/tests/neg/t7093.scala
@@ -1,12 +1,10 @@
-object Test {
-
-  trait A[+X] {
-    protected[this] def f(x: X): X = x
-  }
+trait A[+X] {
+  protected[this] def f(x: X): X = x // error: variance
+}
 
-  trait B extends A[B] {
-    def kaboom = f(new B {})
-  }
+trait B extends A[B] {
+  def kaboom = f(new B {})
+}
 
 // protected[this] disables variance checking
 // of the signature of `f`.
@@ -16,12 +14,12 @@ object Test {
 // The protected[this] loophole is widely used
 // in the collections, every newBuilder method
 // would fail variance checking otherwise.
-  class C extends B with A[C] {
-    override protected[this] def f(c: C) = c
-  }
+class C extends B with A[C] {
+  override protected[this] def f(c: C) = c
+}
 
 // java.lang.ClassCastException: B$$anon$1 cannot be cast to C
 //  at C.f(<console>:15)
+object Test extends App {
   new C().kaboom
 }
-
diff --git a/tests/pos-special/strawman-collections/CollectionStrawMan5.scala b/tests/pos-special/strawman-collections/CollectionStrawMan5.scala
index e97051d2bccf..8fea155ed708 100644
--- a/tests/pos-special/strawman-collections/CollectionStrawMan5.scala
+++ b/tests/pos-special/strawman-collections/CollectionStrawMan5.scala
@@ -56,7 +56,7 @@ object CollectionStrawMan5 {
 
   /** Base trait for strict collections */
   trait Buildable[+A, +To <: Iterable[A]] extends Iterable[A] {
-    protected[this] def newBuilder: Builder[A, To]
+    protected[this] def newBuilder: Builder[A @uncheckedVariance, To]
     override def partition(p: A => Boolean): (To, To) = {
       val l, r = newBuilder
       iterator.foreach(x => (if (p(x)) l else r) += x)
@@ -95,7 +95,7 @@ object CollectionStrawMan5 {
        with IterableOps[A]
        with IterableMonoTransforms[A, C[A @uncheckedVariance]] // sound bcs of VarianceNote
        with IterablePolyTransforms[A, C] {
-    protected[this] def fromLikeIterable(coll: Iterable[A]): C[A] = fromIterable(coll)
+    protected[this] def fromLikeIterable(coll: Iterable[A @uncheckedVariance]): C[A @uncheckedVariance] = fromIterable(coll)
   }
 
   /** Base trait for Seq operations */
@@ -115,7 +115,7 @@ object CollectionStrawMan5 {
 
   trait IterableMonoTransforms[+A, +Repr] extends Any {
     protected def coll: Iterable[A]
-    protected[this] def fromLikeIterable(coll: Iterable[A]): Repr
+    protected[this] def fromLikeIterable(coll: Iterable[A] @scala.annotation.unchecked.uncheckedVariance): Repr
     def filter(p: A => Boolean): Repr = fromLikeIterable(View.Filter(coll, p))
     def partition(p: A => Boolean): (Repr, Repr) = {
       val pn = View.Partition(coll, p)
@@ -166,7 +166,7 @@ object CollectionStrawMan5 {
     }
     def length: Int =
       if (isEmpty) 0 else 1 + tail.length
-    protected[this] def newBuilder = new ListBuffer[A]
+    protected[this] def newBuilder = new ListBuffer[A @uncheckedVariance]
     def ++:[B >: A](prefix: List[B]): List[B] =
       if (prefix.isEmpty) this
       else Cons(prefix.head, prefix.tail ++: this)
diff --git a/tests/pos-special/strawman-collections/CollectionStrawMan6.scala b/tests/pos-special/strawman-collections/CollectionStrawMan6.scala
index 14b2bbae0d1b..a2ff42ba783e 100644
--- a/tests/pos-special/strawman-collections/CollectionStrawMan6.scala
+++ b/tests/pos-special/strawman-collections/CollectionStrawMan6.scala
@@ -168,7 +168,7 @@ object CollectionStrawMan6 extends LowPriority {
   trait Buildable[+A, +Repr] extends Any with IterableMonoTransforms[A, Repr]  {
 
     /** Creates a new builder. */
-    protected[this] def newBuilder: Builder[A, Repr]
+    protected[this] def newBuilder: Builder[A, Repr] @uncheckedVariance
 
     /** Optimized, push-based version of `partition`. */
     override def partition(p: A => Boolean): (Repr, Repr) = {
@@ -226,7 +226,7 @@ object CollectionStrawMan6 extends LowPriority {
     /** Create a collection of type `C[A]` from the elements of `coll`, which has
      *  the same element type as this collection. Overridden in StringOps and ArrayOps.
      */
-    protected[this] def fromIterableWithSameElemType(coll: Iterable[A]): C[A] = fromIterable(coll)
+    protected[this] def fromIterableWithSameElemType(coll: Iterable[A] @uncheckedVariance): C[A]@uncheckedVariance  = fromIterable(coll)
   }
 
   /** Base trait for Seq operations */
@@ -349,7 +349,7 @@ object CollectionStrawMan6 extends LowPriority {
    */
   trait IterableMonoTransforms[+A, +Repr] extends Any {
     protected def coll: Iterable[A]
-    protected[this] def fromIterableWithSameElemType(coll: Iterable[A]): Repr
+    protected[this] def fromIterableWithSameElemType(coll: Iterable[A] @uncheckedVariance): Repr
 
     /** All elements satisfying predicate `p` */
     def filter(p: A => Boolean): Repr = fromIterableWithSameElemType(View.Filter(coll, p))
@@ -420,7 +420,7 @@ object CollectionStrawMan6 extends LowPriority {
 
     def fromIterable[B](c: Iterable[B]): List[B] = List.fromIterable(c)
 
-    protected[this] def newBuilder = new ListBuffer[A].mapResult(_.toList)
+    protected[this] def newBuilder = new ListBuffer[A].mapResult(_.toList): @uncheckedVariance
 
     /** Prepend element */
     def :: [B >: A](elem: B): List[B] =  new ::(elem, this)
diff --git a/tests/pos/GenericTraversableTemplate.scala b/tests/pos/GenericTraversableTemplate.scala
index 70e2376d066f..e9120e33cb69 100644
--- a/tests/pos/GenericTraversableTemplate.scala
+++ b/tests/pos/GenericTraversableTemplate.scala
@@ -62,7 +62,7 @@ trait GenericTraversableTemplate[+A, +CC[X] <: GenTraversable[X]] extends HasNew
 
   /** The builder that builds instances of type $Coll[A]
    */
-  protected[this] def newBuilder: Builder[A, CC[A]] = companion.newBuilder[A]
+  protected[this] def newBuilder: Builder[A, CC[A]] @uncheckedVariance = companion.newBuilder[A]
 
   /** The generic builder that builds instances of $Coll
    *  at arbitrary element types.
diff --git a/tests/pos/t3272.scala b/tests/pos/t3272.scala
deleted file mode 100644
index cf54d6a848fe..000000000000
--- a/tests/pos/t3272.scala
+++ /dev/null
@@ -1,8 +0,0 @@
-trait A {
-  trait C[+T] {
-    protected[this] def f(t: T): Unit = {}
-  }
-  trait D[T] extends C[T] {
-    def g(t: T): Unit = { f(t) }
-  }
-}
diff --git a/tests/pos/t4345.scala b/tests/pos/t4345.scala
deleted file mode 100644
index b0131d5fa5f5..000000000000
--- a/tests/pos/t4345.scala
+++ /dev/null
@@ -1,7 +0,0 @@
-trait C1[+A, +CC[X]] {
-  protected[this] def f: A => CC[A] = sys.error("")
-}
-
-trait C2[+A, +CC[X]] extends C1[A, CC] {
-  override protected[this] def f = super.f
-}
diff --git a/tests/pos/variances-local.scala b/tests/pos/variances-local.scala
deleted file mode 100644
index 35e395095cf1..000000000000
--- a/tests/pos/variances-local.scala
+++ /dev/null
@@ -1,7 +0,0 @@
-class Foo1[+T] {
-  private[this] type MyType = T
-}
-
-class Foo2[+T] {
-  protected[this] type MyType = T
-}
diff --git a/tests/run/CollectionTests.scala b/tests/run/CollectionTests.scala
index 3c08bee0e874..c9d2a2bbb51d 100644
--- a/tests/run/CollectionTests.scala
+++ b/tests/run/CollectionTests.scala
@@ -55,7 +55,7 @@ object CollectionStrawMan5 {
 
   /** Base trait for strict collections */
   trait Buildable[+A, +To <: Iterable[A]] extends Iterable[A] {
-    protected[this] def newBuilder: Builder[A, To]
+    protected[this] def newBuilder: Builder[A @uncheckedVariance, To]
     override def partition(p: A => Boolean): (To, To) = {
       val l, r = newBuilder
       iterator.foreach(x => (if (p(x)) l else r) += x)
@@ -94,7 +94,7 @@ object CollectionStrawMan5 {
        with IterableOps[A]
        with IterableMonoTransforms[A, C[A @uncheckedVariance]] // sound bcs of VarianceNote
        with IterablePolyTransforms[A, C] {
-    protected[this] def fromLikeIterable(coll: Iterable[A]): C[A] = fromIterable(coll)
+    protected[this] def fromLikeIterable(coll: Iterable[A @uncheckedVariance]): C[A @uncheckedVariance] = fromIterable(coll)
   }
 
   /** Base trait for Seq operations */
@@ -114,7 +114,7 @@ object CollectionStrawMan5 {
 
   trait IterableMonoTransforms[+A, +Repr] extends Any {
     protected def coll: Iterable[A]
-    protected[this] def fromLikeIterable(coll: Iterable[A]): Repr
+    protected[this] def fromLikeIterable(coll: Iterable[A @uncheckedVariance ]): Repr
     def filter(p: A => Boolean): Repr = fromLikeIterable(View.Filter(coll, p))
     def partition(p: A => Boolean): (Repr, Repr) = {
       val pn = View.Partition(coll, p)
diff --git a/tests/run/colltest5/CollectionStrawMan5_1.scala b/tests/run/colltest5/CollectionStrawMan5_1.scala
index 183f24bb166b..620359b5c98e 100644
--- a/tests/run/colltest5/CollectionStrawMan5_1.scala
+++ b/tests/run/colltest5/CollectionStrawMan5_1.scala
@@ -48,7 +48,7 @@ object CollectionStrawMan5 {
 
   /** Base trait for strict collections */
   trait Buildable[+A, +To <: Iterable[A]] extends Iterable[A] {
-    protected[this] def newBuilder: Builder[A, To]
+    protected[this] def newBuilder: Builder[A, To] @uncheckedVariance
     override def partition(p: A => Boolean): (To, To) = {
       val l, r = newBuilder
       iterator.foreach(x => (if (p(x)) l else r) += x)
@@ -87,7 +87,7 @@ object CollectionStrawMan5 {
        with IterableOps[A]
        with IterableMonoTransforms[A, C[A @uncheckedVariance]] // sound bcs of VarianceNote
        with IterablePolyTransforms[A, C] {
-    protected[this] def fromLikeIterable(coll: Iterable[A]): C[A] = fromIterable(coll)
+    protected[this] def fromLikeIterable(coll: Iterable[A] @uncheckedVariance): C[A] @uncheckedVariance = fromIterable(coll)
   }
 
   /** Base trait for Seq operations */
@@ -107,7 +107,7 @@ object CollectionStrawMan5 {
 
   trait IterableMonoTransforms[+A, +Repr] extends Any {
     protected def coll: Iterable[A]
-    protected[this] def fromLikeIterable(coll: Iterable[A]): Repr
+    protected[this] def fromLikeIterable(coll: Iterable[A] @uncheckedVariance): Repr
     def filter(p: A => Boolean): Repr = fromLikeIterable(View.Filter(coll, p))
     def partition(p: A => Boolean): (Repr, Repr) = {
       val pn = View.Partition(coll, p)
@@ -158,7 +158,7 @@ object CollectionStrawMan5 {
     }
     def length: Int =
       if (isEmpty) 0 else 1 + tail.length
-    protected[this] def newBuilder = new ListBuffer[A]
+    protected[this] def newBuilder = new ListBuffer[A] @uncheckedVariance
     def ++:[B >: A](prefix: List[B]): List[B] =
       if (prefix.isEmpty) this
       else Cons(prefix.head, prefix.tail ++: this)
diff --git a/tests/run/colltest6/CollectionStrawMan6_1.scala b/tests/run/colltest6/CollectionStrawMan6_1.scala
index 17c28d74ef0f..4fad57154775 100644
--- a/tests/run/colltest6/CollectionStrawMan6_1.scala
+++ b/tests/run/colltest6/CollectionStrawMan6_1.scala
@@ -169,7 +169,7 @@ object CollectionStrawMan6 extends LowPriority {
   trait Buildable[+A, +Repr] extends Any with IterableMonoTransforms[A, Repr]  {
 
     /** Creates a new builder. */
-    protected[this] def newBuilder: Builder[A, Repr]
+    protected[this] def newBuilder: Builder[A @uncheckedVariance, Repr]
 
     /** Optimized, push-based version of `partition`. */
     override def partition(p: A => Boolean): (Repr, Repr) = {
@@ -227,7 +227,7 @@ object CollectionStrawMan6 extends LowPriority {
     /** Create a collection of type `C[A]` from the elements of `coll`, which has
      *  the same element type as this collection. Overridden in StringOps and ArrayOps.
      */
-    protected[this] def fromIterableWithSameElemType(coll: Iterable[A]): C[A] = fromIterable(coll)
+    protected[this] def fromIterableWithSameElemType(coll: Iterable[A @uncheckedVariance]): C[A @uncheckedVariance] = fromIterable(coll)
   }
 
   /** Base trait for Seq operations */
@@ -350,7 +350,7 @@ object CollectionStrawMan6 extends LowPriority {
    */
   trait IterableMonoTransforms[+A, +Repr] extends Any {
     protected def coll: Iterable[A]
-    protected[this] def fromIterableWithSameElemType(coll: Iterable[A]): Repr
+    protected[this] def fromIterableWithSameElemType(coll: Iterable[A @uncheckedVariance]): Repr
 
     /** All elements satisfying predicate `p` */
     def filter(p: A => Boolean): Repr = fromIterableWithSameElemType(View.Filter(coll, p))
@@ -421,7 +421,7 @@ object CollectionStrawMan6 extends LowPriority {
 
     def fromIterable[B](c: Iterable[B]): List[B] = List.fromIterable(c)
 
-    protected[this] def newBuilder = new ListBuffer[A].mapResult(_.toList)
+    protected[this] def newBuilder = new ListBuffer[A @uncheckedVariance].mapResult(_.toList)
 
     /** Prepend element */
     def :: [B >: A](elem: B): List[B] =  new ::(elem, this)

From 1082eb564373dc2111c5cfbf67842dd58e2fda96 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Fri, 16 Mar 2018 17:59:58 +0100
Subject: [PATCH 8/9] Disallow extending a trait twice in the same class

---
 compiler/src/dotty/tools/dotc/typer/Typer.scala |  4 ++++
 tests/neg/i3989e.scala                          | 11 +++++++++++
 2 files changed, 15 insertions(+)
 create mode 100644 tests/neg/i3989e.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index f4ae59e887cb..5e183da83fef 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1460,9 +1460,13 @@ class Typer extends Namer
         ref
     }
 
+    val seenParents = mutable.Set[Symbol]()
+
     def typedParent(tree: untpd.Tree): Tree = {
       var result = if (tree.isType) typedType(tree)(superCtx) else typedExpr(tree)(superCtx)
       val psym = result.tpe.typeSymbol
+      if (seenParents.contains(psym)) ctx.error(i"$psym is extended twice", tree.pos)
+      seenParents += psym
       if (tree.isType) {
         if (psym.is(Trait) && !cls.is(Trait) && !cls.superClass.isSubClass(psym))
           result = maybeCall(result, psym, psym.primaryConstructor.info)
diff --git a/tests/neg/i3989e.scala b/tests/neg/i3989e.scala
new file mode 100644
index 000000000000..33b5ee665815
--- /dev/null
+++ b/tests/neg/i3989e.scala
@@ -0,0 +1,11 @@
+object Test extends App {
+  trait A[+X](val x: X)
+  class B extends A(5) with A("hello") // error: A is extended twice
+
+  def f(a: A[Int]): Int = a match {
+    case b: B => b.x
+    case _ => 0
+  }
+
+  f(new B)
+}
\ No newline at end of file

From 4feda4e0161c13698cf94a84d33349db55a10d60 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Fri, 16 Mar 2018 18:10:03 +0100
Subject: [PATCH 9/9] Enforce non-variant refinement for parameterized base
 classes and traits

---
 .../dotty/tools/dotc/typer/Inferencing.scala  |  3 +-
 .../dotty/tools/dotc/typer/RefChecks.scala    | 63 ++++++++++++++-----
 tests/neg/i3989f.scala                        | 13 ++++
 tests/neg/i3989g.scala                        | 12 ++++
 4 files changed, 76 insertions(+), 15 deletions(-)
 create mode 100644 tests/neg/i3989f.scala
 create mode 100644 tests/neg/i3989g.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
index c5caf8d8a0ba..9b6c128d023b 100644
--- a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
@@ -190,7 +190,8 @@ object Inferencing {
     def refinementIsInvariant(tp: Type): Boolean = tp match {
       case tp: ClassInfo => tp.cls.is(Final) || tp.cls.is(Case)
       case tp: TypeProxy => refinementIsInvariant(tp.underlying)
-      case tp: AndOrType => refinementIsInvariant(tp.tp1) && refinementIsInvariant(tp.tp2)
+      case tp: AndType => refinementIsInvariant(tp.tp1) && refinementIsInvariant(tp.tp2)
+      case tp: OrType => refinementIsInvariant(tp.tp1) && refinementIsInvariant(tp.tp2)
       case _ => false
     }
 
diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index 1f1b12d8e211..a0ad815fa6e1 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -647,21 +647,11 @@ object RefChecks {
        */
       def checkCaseClassInheritanceInvariant() = {
         for (caseCls <- clazz.info.baseClasses.tail.find(_.is(Case)))
-          for (bc <- caseCls.info.baseClasses.tail)
-            if (bc.typeParams.exists(_.paramVariance != 0)) {
-              val caseBT = self.baseType(caseCls)
-              val thisBT = self.baseType(bc)
-              val combinedBT = caseBT.baseType(bc)
-              if (!(thisBT =:= combinedBT))
-                ctx.errorOrMigrationWarning(
-                  em"""illegal inheritance: $clazz inherits case $caseCls
-                      |but the two have different base type instances for $bc.
-                      |
-                      |  Basetype for $clazz: $thisBT
-                      |  Basetype via $caseCls: $combinedBT""", clazz.pos)
-            }
+          for (baseCls <- caseCls.info.baseClasses.tail)
+            if (baseCls.typeParams.exists(_.paramVariance != 0))
+              for (problem <- variantInheritanceProblems(baseCls, caseCls, "non-variant", "case "))
+                ctx.errorOrMigrationWarning(problem(), clazz.pos)
       }
-
       checkNoAbstractMembers()
       if (abstractErrors.isEmpty)
         checkNoAbstractDecls(clazz)
@@ -684,6 +674,51 @@ object RefChecks {
       }
     }
 
+    if (!clazz.is(Trait)) {
+      // check that parameterized base classes and traits are typed in the same way as from the superclass
+      // I.e. say we have
+      //
+      //    Sub extends Super extends* Base
+      //
+      // where `Base` has value parameters. Enforce that
+      //
+      //    Sub.thisType.baseType(Base)  =:=  Sub.thisType.baseType(Super).baseType(Base)
+      //
+      // This is necessary because parameter values are determined directly or indirectly
+      // by `Super`. So we cannot pretend they have a different type when seen from `Sub`.
+      def checkParameterizedTraitsOK() = {
+        val mixins = clazz.mixins
+        for {
+          cls <- clazz.info.baseClasses.tail
+          if cls.paramAccessors.nonEmpty && !mixins.contains(cls)
+          problem <- variantInheritanceProblems(cls, clazz.asClass.superClass, "parameterized", "super")
+        } ctx.error(problem(), clazz.pos)
+      }
+
+      checkParameterizedTraitsOK()
+    }
+
+    /** Check that `site` does not inherit conflicting generic instances of `baseCls`,
+     *  when doing a direct base type or going via intermediate class `middle`. I.e, we require:
+     *
+     *     site.baseType(baseCls)  =:=  site.baseType(middle).baseType(baseCls)
+     *
+     *  Return an optional by name error message if this test fails.
+     */
+    def variantInheritanceProblems(
+        baseCls: Symbol, middle: Symbol, baseStr: String, middleStr: String): Option[() => String] = {
+      val superBT = self.baseType(middle)
+      val thisBT = self.baseType(baseCls)
+      val combinedBT = superBT.baseType(baseCls)
+      if (combinedBT =:= thisBT) None // ok
+      else
+        Some(() =>
+          em"""illegal inheritance: $clazz inherits conflicting instances of $baseStr base $baseCls.
+              |
+              |  Direct basetype: $thisBT
+              |  Basetype via $middleStr$middle: $combinedBT""")
+    }
+
     /* Returns whether there is a symbol declared in class `inclazz`
        * (which must be different from `clazz`) whose name and type
        * seen as a member of `class.thisType` matches `member`'s.
diff --git a/tests/neg/i3989f.scala b/tests/neg/i3989f.scala
new file mode 100644
index 000000000000..ce68f16b54b0
--- /dev/null
+++ b/tests/neg/i3989f.scala
@@ -0,0 +1,13 @@
+object Test extends App {
+  trait A[+X](val x: X)
+  class B[+X](val y: X) extends A[X](y)
+  class C extends B(5) with A[String] // error: illegal inheritance
+
+  class D extends B(5) with A[Any] // ok
+
+  def f(a: A[Int]): String = a match {
+    case c: C => c.x
+    case _ => "hello"
+  }
+  f(new C)
+}
\ No newline at end of file
diff --git a/tests/neg/i3989g.scala b/tests/neg/i3989g.scala
new file mode 100644
index 000000000000..fe42b179effb
--- /dev/null
+++ b/tests/neg/i3989g.scala
@@ -0,0 +1,12 @@
+object Test extends App {
+  class A[+X](val x: X)
+  class B[+X](val y: X) extends A[X](y)
+  trait T[+X] extends A[X]
+  class C extends B(5) with T[String] // error: illegal inheritance
+
+  def f(a: A[Int]): String = a match {
+    case c: C => c.x
+    case _ => "hello"
+  }
+  f(new C)
+}
\ No newline at end of file