From 9d46ce1ee7da8f3a3fc9fe4d5c9b9cb6b58242c4 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Mon, 18 Mar 2019 10:51:35 +0100
Subject: [PATCH 1/2] Fix 6057: Be more forgiving in LazyRef checking if errors
 were reported previously

---
 compiler/src/dotty/tools/dotc/core/Types.scala | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 57164795b402..1e5e53a0ab14 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -2438,7 +2438,14 @@ object Types {
     private[this] var myRef: Type = null
     private[this] var computed = false
     def ref(implicit ctx: Context): Type = {
-      if (computed) assert(myRef != null)
+      if (computed) {
+        if (myRef == null) {
+          // if errors were reported previously handle this by throwing a CyclicReference
+          // instead of crashing immediately. A test case is neg/i6057.scala.
+          assert(ctx.reporter.errorsReported)
+          CyclicReference(NoDenotation)
+        }
+      }
       else {
         computed = true
         myRef = refFn(ctx)

From 67a0cecdf206ef7e9e23a61bfbd61048bc6cc1fd Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Mon, 18 Mar 2019 10:53:12 +0100
Subject: [PATCH 2/2] Add test case

---
 tests/neg/i6057.scala | 4 ++++
 1 file changed, 4 insertions(+)
 create mode 100644 tests/neg/i6057.scala

diff --git a/tests/neg/i6057.scala b/tests/neg/i6057.scala
new file mode 100644
index 000000000000..7816e7b70908
--- /dev/null
+++ b/tests/neg/i6057.scala
@@ -0,0 +1,4 @@
+class i0[i1] {
+    type i2 <: i0[i2]
+    val i3: i2[i4] {}  // error
+}
\ No newline at end of file