Fix optional type typecheck to accept nil

SigTyOptionalNode#typecheck was delegating directly to the inner
type's typecheck, which would reject nil values for T? types like
Binding?. Now it skips nil types when checking, since nil is always
valid for optional types.

This fixes false "wrong type of arguments" for calls like
eval("code", nil, "filename") where the second argument is Binding?.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mame

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