Fix checkNonCyclic.

Need to also look info refined types.
Need to handle case where we hit a NoCompleter again.

Fixes #974 and makes MutableSortedSetFactory in stdlib compile.

Author: odersky

src/dotty/tools/dotc/typer/Checking.scala

@@ -138,20 +138,20 @@ object Checking {
             case SuperType(thistp, _) => isInteresting(thistp)
             case AndType(tp1, tp2) => isInteresting(tp1) || isInteresting(tp2)
             case OrType(tp1, tp2) => isInteresting(tp1) && isInteresting(tp2)
-            case _: RefinedType => false
-              // Note: it's important not to visit parents of RefinedTypes,
-              // since otherwise spurious #Apply projections might be inserted.
+            case _: RefinedType => true
             case _ => false
           }
           // If prefix is interesting, check info of typeref recursively, marking the referred symbol
           // with NoCompleter. This provokes a CyclicReference when the symbol
           // is hit again. Without this precaution we could stackoverflow here.
           if (isInteresting(pre)) {
             val info = tp.info
-            val symInfo = tp.symbol.info
-            if (tp.symbol.exists) tp.symbol.info = SymDenotations.NoCompleter
+            val sym = tp.symbol
+            if (sym.infoOrCompleter == SymDenotations.NoCompleter) throw CyclicReference(sym)
+            val symInfo = sym.info
+            if (sym.exists) sym.info = SymDenotations.NoCompleter
             try checkInfo(info)
-            finally if (tp.symbol.exists) tp.symbol.info = symInfo
+            finally if (sym.exists) sym.info = symInfo
           }
           tp
         } catch {

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

test/dotc/tests.scala

@@ -158,6 +158,7 @@ class tests extends CompilerTest {
   @Test def neg_finalSealed = compileFile(negDir, "final-sealed", xerrors = 2)
   @Test def neg_i705 = compileFile(negDir, "i705-inner-value-class", xerrors = 7)
   @Test def neg_i866 = compileFile(negDir, "i866", xerrors = 2)
+  @Test def neg_i974 = compileFile(negDir, "i974", xerrors = 2)
   @Test def neg_moduleSubtyping = compileFile(negDir, "moduleSubtyping", xerrors = 4)
   @Test def neg_escapingRefs = compileFile(negDir, "escapingRefs", xerrors = 2)
   @Test def neg_instantiateAbstract = compileFile(negDir, "instantiateAbstract", xerrors = 8)

tests/neg/i974.scala

@@ -0,0 +1,8 @@
+trait Foo[T <: Bar[T]#Elem] // error: illegal cyclic reference
+trait Bar[T] {
+  type Elem = T
+}
+trait Foo2[T <: Bar2[T]#Elem] // error: illegal cyclic reference
+trait Bar2[T] {
+  type Elem = T
+}

tests/pos/i974.scala

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