fix optional block arguments in rhs

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

Import ruby/ruby@01740f0c27

src/parse.y

@@ -3065,10 +3065,9 @@ f_opt           : f_opt_asgn arg_value
                     }
                 ;
 
-f_block_opt     : tIDENTIFIER '=' primary_value
+f_block_opt     : f_opt_asgn primary_value
                     {
-                      local_add_f(p, $1);
-                      $$ = cons(nsym($1), $3);
+                      $$ = cons(nsym($1), $2);
                     }
                 ;
 

test/t/syntax.rb

@@ -289,3 +289,7 @@ assert('optional argument in the rhs default expressions') do
   assert_nil(o.t)
   assert_equal("method called", o.t2)
 end
+
+assert('optional block argument in the rhs default expressions') do
+  assert_nil(proc {|foo = foo| foo}.call)
+end