Add scenarios for the splat fallback on union types

Cover the cases the union fallback has to get right: nil must not be
wrapped (`[*nil]` is `[]`), `to_ary` must not stand in for `to_a`, and
adding or removing `to_a` must flip the result, which exercises the
dependency edge that the fallback relies on.

Author: mame

scenario/array/splat.rb

@@ -24,3 +24,37 @@ def foo(x)
 class Object
   def foo: (:sym | [:int]) -> Array[:int | :sym]
 end
+
+## update
+def foo(x)
+  [*x]
+end
+
+foo([1])
+foo(nil)
+
+## assert
+class Object
+  def foo: ([Integer]?) -> Array[Integer]
+end
+
+## update
+class C
+  def to_ary
+    [1]
+  end
+end
+
+def foo(x)
+  [*x]
+end
+
+foo(C.new)
+
+## assert
+class C
+  def to_ary: -> [Integer]
+end
+class Object
+  def foo: (C) -> Array[C]
+end

scenario/incremental/splat-to-a.rb

@@ -0,0 +1,54 @@
+## update
+class C
+end
+
+def foo(x)
+  [*x]
+end
+
+foo(C.new)
+
+## assert
+class C
+end
+class Object
+  def foo: (C) -> Array[C]
+end
+
+## update
+class C
+  def to_a
+    [1]
+  end
+end
+
+def foo(x)
+  [*x]
+end
+
+foo(C.new)
+
+## assert
+class C
+  def to_a: -> [Integer]
+end
+class Object
+  def foo: (C) -> Array[Integer]
+end
+
+## update
+class C
+end
+
+def foo(x)
+  [*x]
+end
+
+foo(C.new)
+
+## assert
+class C
+end
+class Object
+  def foo: (C) -> Array[C]
+end