Don't force NamedType denotations in containsRefinedThis

containsRefinedThis inspects symbols and infos of named types in order
to avoid needless traversals. As i974 shows, this can lead to infinite
recursions. The fix is not to force a NamedType denotation, assume the
worst and traverse the prefix if a NamedType is not yet populated
with a denotation.

Fixes scala#974 and makes MutableSortedSetFactory in stdlib compile.

Author: odersky

src/dotty/tools/dotc/core/TypeApplications.scala

@@ -695,11 +695,13 @@ class TypeApplications(val self: Type) extends AnyVal {
       case RefinedThis(tp) =>
         tp eq target
       case tp: NamedType =>
-        if (tp.symbol.isClass) !tp.symbol.isStatic && recur(tp.prefix)
-        else tp.info match {
-          case TypeAlias(alias) => recur(alias)
-          case _ => recur(tp.prefix)
-        }
+        if (tp.denotationIsCurrent)
+          if (tp.symbol.isClass) !tp.symbol.isStatic && recur(tp.prefix)
+          else tp.info match {
+            case TypeAlias(alias) => recur(alias)
+            case _ => recur(tp.prefix)
+          }
+        else recur(tp.prefix)
       case tp: RefinedType =>
         recur(tp.refinedInfo) || recur(tp.parent)
       case tp: TypeBounds =>

test/dotc/scala-collections.whitelist

@@ -266,7 +266,7 @@
 ./scala-scala/src/library/scala/collection/generic/ParFactory.scala
 
 # https://github.com/lampepfl/dotty/issues/974 -> @smarter
-#./scala-scala/src/library/scala/collection/generic/MutableSortedSetFactory.scala
+./scala-scala/src/library/scala/collection/generic/MutableSortedSetFactory.scala
 
 # cyclic reference, maybe related to #974 -> @smarter
 #./scala-scala/src/library/scala/collection/generic/ParSetFactory.scala

tests/pos/i974.scala

@@ -0,0 +1,2 @@
+class Foo[A]
+class Bar[CC[X] <: Foo[CC[X]]]