added T is nullable support

docs/generics.md

@@ -109,6 +109,24 @@ Examples (T = A | B):
     B in T            // true
     T is A | B        // true
 
+# Nullability checks for types
+
+Use `is nullable` to check whether a type expression accepts `null`:
+
+    T is nullable
+    T !is nullable
+
+This works with concrete and generic types:
+
+    fun describe<T>(x: T): String = when (T) {
+        nullable -> "nullable"
+        else -> "non-null"
+    }
+
+Equivalent legacy form:
+
+    null is T
+
 # Practical examples
 
     fun acceptInts<T: Int>(xs: List<T>) { }

docs/samples/type_nullability_checks.lyng

@@ -0,0 +1,10 @@
+fun describe<T>(x: T): String = when (T) {
+    nullable -> "nullable"
+    else -> "non-null"
+}
+
+type MaybeInt = Int?
+assert(MaybeInt is nullable)
+assert(!(Int is nullable))
+assertEquals("nullable", describe<Int?>(null))
+assertEquals("non-null", describe<Int>(1))

docs/tutorial.md

@@ -502,6 +502,18 @@ Aliases expand to their underlying type expressions. See `docs/generics.md` for
 
 `Null` is the class of `null`. It is a singleton type and mostly useful for type inference results.
 
+For type expressions, you can check nullability directly:
+
+    T is nullable
+    T !is nullable
+
+This is especially useful in generic code and in `when` over a type parameter:
+
+    fun describe<T>(x: T): String = when (T) {
+        nullable -> "nullable"
+        else -> "non-null"
+    }
+
 ## Type inference
 
 The compiler infers types from: