From 7fa34d0633964f6b7acdaed67f7af8e3e8bcd928 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Fri, 15 Feb 2019 13:53:50 +0100
Subject: [PATCH 1/2] Fix #5844: Use an approximating type map to substitute
 parent type params

Actual arguments could be wildcards, which means a normal substitution will
not work.
---
 .../dotty/tools/dotc/core/Substituters.scala  | 24 +++++++++++++++++
 .../tools/dotc/core/SymDenotations.scala      |  2 +-
 tests/pos/i5844.scala                         | 26 +++++++++++++++++++
 3 files changed, 51 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i5844.scala

diff --git a/compiler/src/dotty/tools/dotc/core/Substituters.scala b/compiler/src/dotty/tools/dotc/core/Substituters.scala
index ab07a4d568fd..cdcaf2b4e3d6 100644
--- a/compiler/src/dotty/tools/dotc/core/Substituters.scala
+++ b/compiler/src/dotty/tools/dotc/core/Substituters.scala
@@ -195,4 +195,28 @@ trait Substituters { this: Context =>
   final class SubstParamsMap(from: BindingType, to: List[Type]) extends DeepTypeMap {
     def apply(tp: Type): Type = substParams(tp, from, to, this)
   }
+
+  /** An approximating substitution that can handle wildcards in the `to` list */
+  final class SubstApproxMap(from: List[Symbol], to: List[Type])(implicit ctx: Context) extends ApproximatingTypeMap {
+    def apply(tp: Type): Type = tp match {
+      case tp: NamedType =>
+        val sym = tp.symbol
+        var fs = from
+        var ts = to
+        while (fs.nonEmpty) {
+          if (fs.head eq sym)
+            return ts.head match {
+              case TypeBounds(lo, hi) => range(lo, hi)
+              case tp1 => tp1
+            }
+          fs = fs.tail
+          ts = ts.tail
+        }
+        if (tp.prefix `eq` NoPrefix) tp else tp.derivedSelect(apply(tp.prefix))
+      case _: ThisType | _: BoundType =>
+        tp
+      case _ =>
+        mapOver(tp)
+    }
+  }
 }
diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index 14dfa73b1a1a..93b04c5bda3e 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -1790,7 +1790,7 @@ object SymDenotations {
                   case LambdaParam(_, _) :: _ =>
                     recur(tp.superType)
                   case tparams: List[Symbol @unchecked] =>
-                    recur(tycon).subst(tparams, args)
+                    new ctx.SubstApproxMap(tparams, args).apply(recur(tycon))
                 }
               record(tp, baseTp)
               baseTp
diff --git a/tests/pos/i5844.scala b/tests/pos/i5844.scala
new file mode 100644
index 000000000000..ca735de7eaf0
--- /dev/null
+++ b/tests/pos/i5844.scala
@@ -0,0 +1,26 @@
+package p1 {
+  trait A
+  trait B[X, Y]{
+    def m(): X
+  }
+  trait C[X] extends B[X, X & A]
+
+  object O{
+    def m(c: C[_]) = {
+      val x = c.m()
+    }
+  }
+}
+package p2 {
+  trait A
+  trait B[X]{
+    def m(): X
+  }
+  trait C[X] extends B[X with A]
+
+  object O{
+    def m(c: C[_]) = {
+      val x: A = c.m()
+    }
+  }
+}
\ No newline at end of file

From cf5d20e9dd15b3d756b3eb3bc45b15fec4a42de1 Mon Sep 17 00:00:00 2001
From: Guillaume Martres <smarter@ubuntu.com>
Date: Fri, 15 Feb 2019 16:57:53 +0100
Subject: [PATCH 2/2] Update
 compiler/src/dotty/tools/dotc/core/Substituters.scala

Co-Authored-By: odersky <odersky@gmail.com>
---
 compiler/src/dotty/tools/dotc/core/Substituters.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/src/dotty/tools/dotc/core/Substituters.scala b/compiler/src/dotty/tools/dotc/core/Substituters.scala
index cdcaf2b4e3d6..f39ec9a33913 100644
--- a/compiler/src/dotty/tools/dotc/core/Substituters.scala
+++ b/compiler/src/dotty/tools/dotc/core/Substituters.scala
@@ -212,7 +212,7 @@ trait Substituters { this: Context =>
           fs = fs.tail
           ts = ts.tail
         }
-        if (tp.prefix `eq` NoPrefix) tp else tp.derivedSelect(apply(tp.prefix))
+        if (tp.prefix `eq` NoPrefix) tp else derivedSelect(tp, apply(tp.prefix))
       case _: ThisType | _: BoundType =>
         tp
       case _ =>