Add bound tests

Author: noti0na1

tests/explicit-nulls/neg/bounds.scala

@@ -0,0 +1,22 @@
+val x1: String = ???
+val x2: String | Null = ???
+
+// T has to be nullable type
+def f1[T >: Null <: AnyRef | Null](x: T): T = x
+
+// Null is no longer a subtype of AnyRef, impossible to apply this method directly.
+// We can bypass this restriction by importing unsafeNulls.
+def f2[T >: Null <: AnyRef](x: T): T = x
+
+def nullOf[T >: Null <: AnyRef | Null]: T = null
+
+def g = {
+  f1(x1)
+  f1(x2)
+
+  f2(x1) // error
+  f2(x2) // error
+
+  val n1: String = nullOf // error
+  val n3: String | Null = nullOf
+}

tests/explicit-nulls/unsafe-common/unsafe-bounds.scala

@@ -0,0 +1,22 @@
+// See explicit-nulls/neg/bounds.scala
+
+val x1: String = ???
+val x2: String | Null = ???
+
+def f1[T >: Null <: AnyRef | Null](x: T): T = x
+def f2[T >: Null <: AnyRef](x: T): T = x
+
+def nullOf[T >: Null <: AnyRef]: T = null
+
+def g = {
+  f1(x1)
+  f1(x2)
+
+  f2[String](x2) // error
+
+  f2(x1) // error
+  f2(x2) // error
+
+  val n1: String = nullOf // error
+  val n2: String | Null = nullOf // ok, Null is inferred
+}

tests/explicit-nulls/unsafe-common/unsafe-bounds2.scala

@@ -0,0 +1,16 @@
+// The bound check is in PostTyper, hence we need a seperate test
+
+// See explicit-nulls/neg/bounds.scala
+
+val x1: String = ???
+val x2: String | Null = ???
+
+def f2[T >: Null <: AnyRef](x: T): T = x
+
+def nullOf[T >: Null <: AnyRef]: T = null
+
+def g = {
+  f2[String](x1) // error
+  f2[String | Null](x1) // error
+  f2[String | Null](x2) // error
+}