From ccd71bb0ffcc640574695e0c58e3e56d750172bd Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 1 Jun 2021 11:37:49 +0200
Subject: [PATCH] Harden type comparer in presence of kind errors

It should not crash when an override has the wrong kind.

Fixes #12664
---
 .../src/dotty/tools/dotc/core/TypeComparer.scala   |  2 +-
 tests/neg/i12664.scala                             | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i12664.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 47f36255de81..7c7aba8a95b0 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1422,7 +1422,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
     def recurArgs(args1: List[Type], args2: List[Type], tparams2: List[ParamInfo]): Boolean =
       if (args1.isEmpty) args2.isEmpty
-      else args2.nonEmpty && {
+      else args2.nonEmpty && tparams2.nonEmpty && {
         val tparam = tparams2.head
         val v = tparam.paramVarianceSign
 
diff --git a/tests/neg/i12664.scala b/tests/neg/i12664.scala
new file mode 100644
index 000000000000..5e4b784475aa
--- /dev/null
+++ b/tests/neg/i12664.scala
@@ -0,0 +1,14 @@
+trait Step {
+  type Self
+  type Next[A]
+}
+
+trait DynamicNextStep {
+  type OneOf[Self, Next[_]]
+  def apply(s: Step): OneOf[s.Self, s.Next]
+}
+
+object X extends DynamicNextStep {
+  override type OneOf[Self] = Self  // error
+  override def apply(s: Step) = ???
+}