Fix substDealias

substDealias did not follow aliases when the prefix of a typeref changed
under substitution. This was exhibited by a bug in extensionMethods which was
first discovered in CollectionStrawMan6 and was minimized in extmethods.

2 files changed, 17 insertions, 7 deletions

diff --git a/src/dotty/tools/dotc/core/Substituters.scala b/src/dotty/tools/dotc/core/Substituters.scala
index 0d1c78e2f..23683608a 100644
--- a/src/dotty/tools/dotc/core/Substituters.scala
+++ b/src/dotty/tools/dotc/core/Substituters.scala
@@ -102,14 +102,13 @@ trait Substituters { this: Context =>
         }
         if (sym.isStatic && !existsStatic(from)) tp
         else {
-          val prefix1 = substDealias(tp.prefix, from, to, theMap)
-          if (prefix1 ne tp.prefix) tp.derivedSelect(prefix1)
-          else if (sym.isAliasType) {
-            val hi = sym.info.bounds.hi
-            val hi1 = substDealias(hi, from, to, theMap)
-            if (hi1 eq hi) tp else hi1
+          tp.info match {
+            case TypeAlias(alias) =>
+              val alias1 = substDealias(alias, from, to, theMap)
+              if (alias1 ne alias) return alias1
+            case _ =>
           }
-          else tp
+          tp.derivedSelect(substDealias(tp.prefix, from, to, theMap))
         }
       case _: ThisType | _: BoundType | NoPrefix =>
         tp
diff --git a/tests/pos/extmethods.scala b/tests/pos/extmethods.scala
index cac1c4ec1..fe95a1c79 100644
--- a/tests/pos/extmethods.scala
+++ b/tests/pos/extmethods.scala
@@ -9,3 +9,14 @@ class Foo[+A <: AnyRef](val xs: List[A]) extends AnyVal {
   def baz[B >: A](x: B): List[B] = ???
 }
 
+object CollectionStrawMan {
+  import collection.mutable.ArrayBuffer
+  import reflect.ClassTag
+
+  implicit class ArrayOps[A](val xs: Array[A]) extends AnyVal {
+
+    def elemTag: ClassTag[A] = ClassTag(xs.getClass.getComponentType)
+
+    protected[this] def newBuilder = new ArrayBuffer[A].mapResult(_.toArray(elemTag))
+  }
+}