Fill in lead num for blocks with it

Fixes [Bug #21256]

Co-Authored-By: Earlopain <[email protected]>

Author: kddnewton

prism_compile.c

@@ -6740,6 +6740,12 @@ pm_compile_scope_node(rb_iseq_t *iseq, pm_scope_node_t *scope_node, const pm_nod
         body->param.flags.has_lead = true;
     }
 
+    // Fill in the anonymous `it` parameter, if it exists
+    if (scope_node->parameters && PM_NODE_TYPE_P(scope_node->parameters, PM_IT_PARAMETERS_NODE)) {
+        body->param.lead_num = 1;
+        body->param.flags.has_lead = true;
+    }
+
     //********END OF STEP 3**********
 
     //********STEP 4**********

test/ruby/test_syntax.rb

@@ -2004,6 +2004,19 @@ def test_it
     assert_equal(/9/, eval('9.then { /#{it}/o }'))
   end
 
+  def test_it_with_splat_super_method
+    bug21256 = '[ruby-core:121592] [Bug #21256]'
+
+    a = Class.new do
+      define_method(:foo) { it }
+    end
+    b = Class.new(a) do
+      def foo(*args) = super
+    end
+
+    assert_equal(1, b.new.foo(1), bug21256)
+  end
+
   def test_value_expr_in_condition
     mesg = /void value expression/
     assert_syntax_error("tap {a = (true ? next : break)}", mesg)