fix optional arguments in rhs

define optional arguments as argument variables in the rhs default
expressions, as same as mere assignment expressions.

Import ruby/ruby@01740f0c273c89f7bcff3d5014d73c8ff6fb1986

src/parse.y

@@ -1075,7 +1075,7 @@ heredoc_end(parser_state *p)
 %type <nd> brace_block cmd_brace_block do_block lhs none f_bad_arg
 %type <nd> mlhs mlhs_list mlhs_post mlhs_basic mlhs_item mlhs_node mlhs_inner
 %type <id> fsym sym basic_symbol operation operation2 operation3
-%type <id> cname fname op f_rest_arg f_block_arg opt_f_block_arg f_norm_arg
+%type <id> cname fname op f_rest_arg f_block_arg opt_f_block_arg f_norm_arg f_opt_asgn
 %type <nd> heredoc words symbols
 
 %token tUPLUS             /* unary+ */
@@ -3052,10 +3052,16 @@ f_arg           : f_arg_item
                     }
                 ;
 
-f_opt           : tIDENTIFIER '=' arg_value
+f_opt_asgn      : tIDENTIFIER '='
                     {
                       local_add_f(p, $1);
-                      $$ = cons(nsym($1), $3);
+                      $$ = $1;
+                    }
+                ;
+
+f_opt           : f_opt_asgn arg_value
+                    {
+			$$ = cons(nsym($1), $2);
                     }
                 ;
 

test/t/syntax.rb

@@ -272,3 +272,20 @@ assert('method definition in cmdarg') do
   end
   true
 end
+
+assert('optional argument in the rhs default expressions') do
+  class OptArgInRHS
+    def foo
+      "method called"
+    end
+    def t(foo = foo)
+      foo
+    end
+    def t2(foo = foo())
+      foo
+    end
+  end
+  o = OptArgInRHS.new
+  assert_nil(o.t)
+  assert_equal("method called", o.t2)
+end