Merge pull request scala#13604 from dotty-staging/fix-11631

add tests for scala#11631

Author: OlivierBlanvillain

tests/pos/i11631.scala

@@ -0,0 +1,19 @@
+trait MyTrait:
+  def a(): String = ""
+
+class Nul
+
+extension [T](x: T | Nul) inline def nnn: x.type & T = ???
+
+class MyClass:
+  var myTrait: MyTrait|Null = null
+
+  def printA(): Unit = println(myTrait.nnn.a())
+
+@main def runTest(): Unit =
+  val mt = new MyTrait:
+    override def a(): String = "hello world"
+
+  val mc = MyClass()
+  mc.myTrait = mt
+  mc.printA()

tests/pos/i11631b.scala

@@ -0,0 +1,15 @@
+trait MyTrait:
+  def a(): String = ""
+
+class MyClass:
+  var myTrait: MyTrait|Null = null
+
+  def printA(): Unit = println(myTrait.nn.a())
+
+@main def runTest(): Unit =
+  val mt = new MyTrait:
+    override def a(): String = "hello world"
+
+  val mc = MyClass()
+  mc.myTrait = mt
+  mc.printA()