From 9482449322cd374f783d4ffa28d1a3a32f9b8f90 Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Wed, 28 Aug 2024 16:37:05 +0200
Subject: [PATCH] Fix canComparePredefined(Nothing, T) in explicit nulls

[Cherry-picked 2f9f371d5bd5a136eec134d1a2242121efb31c81]
---
 .../src/dotty/tools/dotc/typer/Synthesizer.scala |  3 ++-
 tests/explicit-nulls/pos/i21392.scala            | 16 ++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 tests/explicit-nulls/pos/i21392.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala b/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
index f19f97e24c92..dca270f683c9 100644
--- a/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala
@@ -186,7 +186,8 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
         // val x: String = null.asInstanceOf[String]
         // if (x == null) {} // error: x is non-nullable
         // if (x.asInstanceOf[String|Null] == null) {} // ok
-        cls1 == defn.NullClass && cls1 == cls2
+        if cls1 == defn.NullClass || cls2 == defn.NullClass then cls1 == cls2
+        else cls1 == defn.NothingClass || cls2 == defn.NothingClass
       else if cls1 == defn.NullClass then
         cls1 == cls2 || cls2.derivesFrom(defn.ObjectClass)
       else if cls2 == defn.NullClass then
diff --git a/tests/explicit-nulls/pos/i21392.scala b/tests/explicit-nulls/pos/i21392.scala
new file mode 100644
index 000000000000..0266199b7831
--- /dev/null
+++ b/tests/explicit-nulls/pos/i21392.scala
@@ -0,0 +1,16 @@
+//> using options -language:strictEquality
+
+import scala.collection.LinearSeq
+
+def foo[T](a: LinearSeq[T]) = a match
+  case Nil => -1
+  case head +: tail => head
+
+enum Foo derives CanEqual:
+  case Bar
+  case Baz(x: String)
+
+
+def foo(a: Foo) = a match
+  case Foo.Bar => -1
+  case _ => 0
\ No newline at end of file