[PRISM] Fix numbered parameters stealing local names

Previously, the local index of numbered parameters were assigned to
names of regular locals, making it hard to read both of them. Use proper
`_[1-9]` numbered parameters. This fixes `test_shapes.rb`.

Also, properly mark the iseq as having lead parameters.

prism_compile.c

@@ -7039,10 +7039,15 @@ pm_compile_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *const ret,
         // Fill in any NumberedParameters, if they exist
         if (scope_node->parameters && PM_NODE_TYPE_P(scope_node->parameters, PM_NUMBERED_PARAMETERS_NODE)) {
             int maximum = ((pm_numbered_parameters_node_t *)scope_node->parameters)->maximum;
+            RUBY_ASSERT(0 < maximum && maximum <= 9);
             for (int i = 0; i < maximum; i++, local_index++) {
-                pm_constant_id_t constant_id = locals->ids[i];
+                const uint8_t param_name[] = { '_', '1' + i };
+                pm_constant_id_t constant_id = pm_constant_pool_find(&parser->constant_pool, param_name, 2);
+                RUBY_ASSERT(constant_id && "parser should fill in any gaps in numbered parameters");
                 pm_insert_local_index(constant_id, local_index, index_lookup_table, local_table_for_iseq, scope_node);
             }
+            body->param.lead_num = maximum;
+            body->param.flags.has_lead = true;
         }
         //********END OF STEP 3**********
 

test/ruby/test_compile_prism.rb

@@ -1721,6 +1721,11 @@ a
       assert_prism_eval("def self.foo(a:, b: 2, c: '3'.to_i); [a, b, c]; end; foo(a: 1)")
     end
 
+    def test_numbered_params
+      assert_prism_eval("[1, 2, 3].then { _3 }")
+      assert_prism_eval("1.then { one = 1; one + _1 }")
+    end
+
     def test_rescue_with_ensure
       assert_prism_eval(<<-CODE)
 begin