From e22dfcc085c6342d8ecfb4040114a7326fa6906d Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 19 Feb 2019 22:18:31 +0100
Subject: [PATCH 1/2] Fix #5948: Tighten rule in hasMatchingMember

This is a partial fix only. We still have to tighten rules around capture to
flag the original example as erroneous.
---
 .../dotty/tools/dotc/core/TypeComparer.scala  |  4 ++--
 tests/neg/i5948.scala                         | 20 +++++++++++++++++++
 tests/neg/i5948a.scala                        | 14 +++++++++++++
 3 files changed, 36 insertions(+), 2 deletions(-)
 create mode 100644 tests/neg/i5948.scala
 create mode 100644 tests/neg/i5948a.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 5d2367a633b5..5e455fb5180d 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1132,7 +1132,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
     /*>|>*/ trace(i"hasMatchingMember($tp1 . $name :? ${tp2.refinedInfo}), mbr: ${tp1.member(name).info}", subtyping) /*<|<*/ {
       val rinfo2 = tp2.refinedInfo
 
-      // If the member is an abstract type, compare the member itself
+      // If the member is an abstract type and the prefix is a path, compare the member itself
       // instead of its bounds. This case is needed situations like:
       //
       //    class C { type T }
@@ -1148,7 +1148,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
       def matchAbstractTypeMember(info1: Type) = info1 match {
         case TypeBounds(lo, hi) if lo ne hi =>
           tp2.refinedInfo match {
-            case rinfo2: TypeBounds =>
+            case rinfo2: TypeBounds if tp1.widenExpr.isSingleton =>
               val ref1 = tp1.widenExpr.select(name)
               isSubType(rinfo2.lo, ref1) && isSubType(ref1, rinfo2.hi)
             case _ =>
diff --git a/tests/neg/i5948.scala b/tests/neg/i5948.scala
new file mode 100644
index 000000000000..1f93d585090e
--- /dev/null
+++ b/tests/neg/i5948.scala
@@ -0,0 +1,20 @@
+abstract class Foo {
+  type A
+  var elem: A
+}
+
+object Test {
+  def setFirstInList[T](list: List[Foo { type A = T }]) = {
+    list(0).elem = list(1).elem
+  }
+
+  def main(args: Array[String]): Unit = {
+    val fooInt = new Foo { type A = Int; var elem = 1 }
+    val fooString = new Foo { type A = String; var elem = "" }
+
+    val list: List[Foo] = List(fooInt, fooString)
+    setFirstInList[Foo#A](list) // error
+    setFirstInList(list) // error
+    println(fooInt.elem + 1)
+  }
+}
\ No newline at end of file
diff --git a/tests/neg/i5948a.scala b/tests/neg/i5948a.scala
new file mode 100644
index 000000000000..ed95a0312a80
--- /dev/null
+++ b/tests/neg/i5948a.scala
@@ -0,0 +1,14 @@
+trait Bar {
+  type A
+}
+
+object Test {
+  def test0: Unit = {
+    val b1: Bar = new Bar {}
+    val b2: Bar { type A = Bar#A } = b1 // error
+  }
+  def test1: Unit = {
+    val b1: List[Bar] = List(new Bar {})
+    val b2: List[Bar { type A = Bar#A }] = b1 // error
+  }
+}
\ No newline at end of file

From 4d198ec4c582285b3cf28ef7e612697b1b2ea55b Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Wed, 20 Feb 2019 14:31:39 +0100
Subject: [PATCH 2/2] Fix stability condition in hasMatchingMember

---
 compiler/src/dotty/tools/dotc/core/TypeComparer.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 5e455fb5180d..c7918426d54c 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1148,7 +1148,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
       def matchAbstractTypeMember(info1: Type) = info1 match {
         case TypeBounds(lo, hi) if lo ne hi =>
           tp2.refinedInfo match {
-            case rinfo2: TypeBounds if tp1.widenExpr.isSingleton =>
+            case rinfo2: TypeBounds if tp1.isStable =>
               val ref1 = tp1.widenExpr.select(name)
               isSubType(rinfo2.lo, ref1) && isSubType(ref1, rinfo2.hi)
             case _ =>