Restrict betaReduce to hk applications

Also, handle LazyRefs conservatively in isReferredTo

Without the change we risk losing RecTypes because
it looks like nobody refers to them. This was observed
for pos/i974.scala.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index ab5c795e3..7a050c412 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -2135,7 +2135,7 @@ object Types {
     }
 
     def betaReduce(implicit ctx: Context): Type = refinedInfo match {
-      case TypeAlias(alias) =>
+      case TypeAlias(alias) if refinedName.isHkArgName =>
         def instantiate(rt: RecType) = new TypeMap {
           def apply(t: Type) = t match {
             case TypeRef(RecThis(`rt`), `refinedName`) => alias
@@ -2265,6 +2265,8 @@ object Types {
           tp match {
             case tp: TypeRef => apply(x, tp.prefix)
             case tp: RecThis => RecType.this eq tp.binder
+            case tp: LazyRef => true // Assume a reference to be safe.
+              // TODO: Check that all accumulators handle LazyRefs correctly
             case _ => foldOver(x, tp)
           }
         }