Fix optional type typecheck to accept nil

lib/typeprof/core/ast/sig_type.rb

@@ -884,6 +884,16 @@ def contravariant_vertex0(genv, changes, vtx, subst)
       end
 
       def typecheck(genv, changes, vtx, subst)
+        # For optional type T?, check if all non-nil types match T.
+        # nil is always acceptable.
+        changes.add_edge(genv, vtx, changes.target)
+        has_non_nil = false
+        vtx.each_type do |ty|
+          next if ty.is_a?(Type::Bot)
+          next if ty == genv.nil_type
+          has_non_nil = true
+        end
+        return true unless has_non_nil
         @type.typecheck(genv, changes, vtx, subst)
       end
 

scenario/rbs/optional_nil_typecheck.rb

@@ -0,0 +1,21 @@
+## update: test.rbs
+class Object
+  def bar: (Binding?) -> void
+end
+
+## update: test.rb
+eval("hello", nil, "test")
+
+def bar_nil
+  bar(nil)
+end
+
+def bar_binding
+  bar(binding)
+end
+
+## assert
+class Object
+  def bar_nil: -> Object
+  def bar_binding: -> Object
+end