Fixed duplicated warning

As `command_rhs` is always a "value expression", `command_asgn`
does not need the same check.

parse.y

@@ -1479,7 +1479,6 @@ stmt		: keyword_alias fitem {SET_LEX_STATE(EXPR_FNAME|EXPR_FITEM);} fitem
 command_asgn	: lhs '=' command_rhs
 		    {
 		    /*%%%*/
-			value_expr($3);
 			$$ = node_assign(p, $1, $3, &@$);
 		    /*% %*/
 		    /*% ripper: assign!($1, $3) %*/
@@ -1487,7 +1486,6 @@ command_asgn	: lhs '=' command_rhs
 		| var_lhs tOP_ASGN command_rhs
 		    {
 		    /*%%%*/
-			value_expr($3);
 			$$ = new_op_assign(p, $1, $2, $3, &@$);
 		    /*% %*/
 		    /*% ripper: opassign!($1, $2, $3) %*/
@@ -1495,7 +1493,6 @@ command_asgn	: lhs '=' command_rhs
 		| primary_value '[' opt_call_args rbracket tOP_ASGN command_rhs
 		    {
 		    /*%%%*/
-			value_expr($6);
 			$$ = new_ary_op_assign(p, $1, $3, $5, $6, &@3, &@$);
 		    /*% %*/
 		    /*% ripper: opassign!(aref_field!($1, escape_Qundef($3)), $5, $6) %*/
@@ -1504,7 +1501,6 @@ command_asgn	: lhs '=' command_rhs
 		| primary_value call_op tIDENTIFIER tOP_ASGN command_rhs
 		    {
 		    /*%%%*/
-			value_expr($5);
 			$$ = new_attr_op_assign(p, $1, $2, $3, $4, $5, &@$);
 		    /*% %*/
 		    /*% ripper: opassign!(field!($1, $2, $3), $4, $5) %*/
@@ -1512,7 +1508,6 @@ command_asgn	: lhs '=' command_rhs
 		| primary_value call_op tCONSTANT tOP_ASGN command_rhs
 		    {
 		    /*%%%*/
-			value_expr($5);
 			$$ = new_attr_op_assign(p, $1, $2, $3, $4, $5, &@$);
 		    /*% %*/
 		    /*% ripper: opassign!(field!($1, $2, $3), $4, $5) %*/
@@ -1528,7 +1523,6 @@ command_asgn	: lhs '=' command_rhs
 		| primary_value tCOLON2 tIDENTIFIER tOP_ASGN command_rhs
 		    {
 		    /*%%%*/
-			value_expr($5);
 			$$ = new_attr_op_assign(p, $1, ID2VAL(idCOLON2), $3, $4, $5, &@$);
 		    /*% %*/
 		    /*% ripper: opassign!(field!($1, ID2VAL(idCOLON2), $3), $4, $5) %*/

test/ruby/test_parse.rb

@@ -1166,6 +1166,12 @@ x = __ENCODING__
     assert_valid_syntax('let () { m(a) do; end }')
   end
 
+  def test_void_value_in_command_rhs
+    w = "void value expression"
+    ex = assert_syntax_error("x = return 1", w)
+    assert_equal(1, ex.message.scan(w).size, "same #{w.inspect} warning should be just once")
+  end
+
 =begin
   def test_past_scope_variable
     assert_warning(/past scope/) {catch {|tag| eval("BEGIN{throw tag}; tap {a = 1}; a")}}