From 960164225ddcc531cc57ff6feab3191478a2e75e Mon Sep 17 00:00:00 2001
From: Liu Fengyun <liu@fengy.me>
Date: Tue, 8 Jun 2021 15:49:17 +0200
Subject: [PATCH 1/2] Fix #10900: Avoid loop for F-bounds in checkCanEqual

---
 compiler/src/dotty/tools/dotc/typer/Implicits.scala | 2 ++
 tests/pos/i10900.scala                              | 3 +++
 2 files changed, 5 insertions(+)
 create mode 100644 tests/pos/i10900.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index ae3b04d37657..823e47eceba5 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -929,6 +929,8 @@ trait Implicits:
           apply(t.widen)
         case t: RefinedType =>
           apply(t.parent)
+        case t: LazyRef =>
+          t
         case _ =>
           if (variance > 0) mapOver(t) else t
       }
diff --git a/tests/pos/i10900.scala b/tests/pos/i10900.scala
new file mode 100644
index 000000000000..8f22a1f9991c
--- /dev/null
+++ b/tests/pos/i10900.scala
@@ -0,0 +1,3 @@
+import scala.collection.IterableOps
+def foo[CC[A] <: IterableOps[A, CC, CC[A]], A](collection: CC[A]) =
+  collection == collection

From 87d0ddcbcae4f5fabeed676c0fbeadb08ce5c4af Mon Sep 17 00:00:00 2001
From: Liu Fengyun <liu@fengy.me>
Date: Tue, 8 Jun 2021 15:52:51 +0200
Subject: [PATCH 2/2] Add original tests

---
 tests/pos/i10900.scala | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/tests/pos/i10900.scala b/tests/pos/i10900.scala
index 8f22a1f9991c..6b7754d7e38f 100644
--- a/tests/pos/i10900.scala
+++ b/tests/pos/i10900.scala
@@ -1,3 +1,23 @@
 import scala.collection.IterableOps
 def foo[CC[A] <: IterableOps[A, CC, CC[A]], A](collection: CC[A]) =
   collection == collection
+
+object Test1 {
+  import scala.collection.IterableOps
+  implicit class RichCollection[CC[A] <: IterableOps[A, CC, CC[A]], A](val collection: CC[A]) {
+    def awm(update: CC[A] => CC[A]): CC[A] = {
+      val newCollection = update(collection)
+      if (newCollection == collection) collection else newCollection.awm(update)
+    }
+  }
+}
+
+object Test2 {
+  import scala.collection.IterableOps
+  implicit class RichCollection[CC[A] <: IterableOps[A, CC, CC[A]], A](val collection: CC[A]) {
+    def awm(update: CC[A] => CC[A]): CC[A] = update(collection) match {
+      case `collection` => collection
+      case updated      => updated.awm(update)
+    }
+  }
+}