test/t/syntax.rb: add test for &nil in formal parameters

Co-authored-by: Claude <noreply@anthropic.com>

test/t/syntax.rb

@@ -1271,3 +1271,26 @@ assert('pattern matching - complex patterns') do
     assert_equal [2, 3], rest
   end
 end
+
+assert('&nil in formal parameters') do
+  def m(&nil); end
+  m
+  m(&nil)
+  assert_raise(ArgumentError) { m {} }
+
+  def m2(a, b, &nil); end
+  m2(1, 2)
+  assert_raise(ArgumentError) { m2(1, 2) {} }
+
+  def m3(a, b=1, *c, &nil); end
+  m3(1)
+  assert_raise(ArgumentError) { m3(1) {} }
+
+  def m4(a:, &nil); end
+  m4(a: 1)
+  assert_raise(ArgumentError) { m4(a: 1) {} }
+
+  f = ->(&nil) { :ok }
+  assert_equal :ok, f.call
+  assert_raise(ArgumentError) { f.call {} }
+end