Merge pull request #2751 from dotty-staging/add-check-cycles

Detect direct cycles of typerefs referring to themselves

Author: odersky

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -108,6 +108,11 @@ object Config {
    */
   final val checkMethodTypes = false
 
+  /** If this flag is set, it is checked that TypeRefs don't refer directly
+   *  to themselves.
+   */
+  final val checkTypeRefCycles = false
+
   /** The recursion depth for showing a summarized string */
   final val summarizeDepth = 2
 

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1527,7 +1527,18 @@ object Types {
     }
 
     /** Hook for adding debug check code when denotations are assigned */
-    final def checkDenot()(implicit ctx: Context) = {}
+    final def checkDenot()(implicit ctx: Context) =
+      if (Config.checkTypeRefCycles)
+        lastDenotation match {
+          case d: SingleDenotation =>
+            d.infoOrCompleter match {
+              case TypeBounds(lo, hi) =>
+                assert(lo ne this, this)
+                assert(hi ne this, this)
+              case _ =>
+            }
+          case _ =>
+        }
 
     /** A second fallback to recompute the denotation if necessary */
     private def computeDenot(implicit ctx: Context): Denotation = {
@@ -1563,9 +1574,9 @@ object Types {
 
           // Don't use setDenot here; double binding checks can give spurious failures after erasure
           lastDenotation = d
-          checkDenot()
           lastSymbol = d.symbol
           checkedPeriod = ctx.period
+          checkDenot()
         }
         d
       }