From 9d729c32693b5b7ea673a57f05b20005caf71df9 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Mon, 6 Dec 2021 19:48:33 +0100
Subject: [PATCH] Check variance of RHS of opaque type

Fixes #13997
---
 .../src/dotty/tools/dotc/transform/PostTyper.scala     |  2 ++
 tests/neg/i13997.scala                                 | 10 ++++++++++
 2 files changed, 12 insertions(+)
 create mode 100644 tests/neg/i13997.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/PostTyper.scala b/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
index 048395e8dffa..5d08921f1ea6 100644
--- a/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
+++ b/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
@@ -378,6 +378,8 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           else (tree.rhs, sym.info) match
             case (rhs: LambdaTypeTree, bounds: TypeBounds) =>
               VarianceChecker.checkLambda(rhs, bounds)
+              if sym.isOpaqueAlias then
+                VarianceChecker.checkLambda(rhs, TypeBounds.upper(sym.opaqueAlias))
             case _ =>
           processMemberDef(super.transform(tree))
         case tree: New if isCheckable(tree) =>
diff --git a/tests/neg/i13997.scala b/tests/neg/i13997.scala
new file mode 100644
index 000000000000..77cca35d93bd
--- /dev/null
+++ b/tests/neg/i13997.scala
@@ -0,0 +1,10 @@
+  opaque type CovariantArray[+A] = Array[A] // error
+
+  object CovariantArray:
+    def crash() =
+      val stringArray: CovariantArray[String] = Array("foo", "bar")
+      val anyArray: CovariantArray[Any] = stringArray
+      anyArray(0) = 42
+      stringArray(0).length
+
+  @main def Test = CovariantArray.crash()
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