BUG #15237: I got "ERROR: source for a multiple-column UPDATE itemmust be a sub-SELECT or ROW() expression"

The following bug has been logged on the website:

Bug reference:      15237
Logged by:          zhou xiaowei
Email address:      110876189@qq.com
PostgreSQL version: 11beta1
Operating system:   centos
Description:

On PostgreSQL 11 Beta, I exec sql like "update
fvt_obj_operate_update_table_033 set (c_int) = (20) where c_int = 20;",
which only on column to set, got ""ERROR:  source for a multiple-column
UPDATE item must be a sub-SELECT or ROW() expression"".
But I exec sql like "update fvt_obj_operate_update_table_033 set
(c_int,c_dp) = (20,1.11) where c_int = 20;", got successfull message.
Detail:
postgres=# \d fvt_obj_operate_update_table_033
               Table "public.fvt_obj_operate_update_table_033"
    Column    |            Type             | Collation | Nullable |
Default
--------------+-----------------------------+-----------+----------+---------
 c_char       | character(102400)           |           |          |
 c_varchar    | character varying(1024)     |           |          |
 c_int        | integer                     |           |          |
 c_dp         | double precision            |           |          |
 c_ts_without | timestamp without time zone |           |          |
Indexes:
    "fvt_obj_operate_update_index_033" UNIQUE, btree (c_int)

postgres=# update fvt_obj_operate_update_table_033 set (c_int) = (20) where
c_int = 20;
2018-06-12 09:46:39.877 CST [94901] ERROR:  source for a multiple-column
UPDATE item must be a sub-SELECT or ROW() expression at character 56
2018-06-12 09:46:39.877 CST [94901] STATEMENT:  update
fvt_obj_operate_update_table_033 set (c_int) = (20) where c_int = 20;
ERROR:  source for a multiple-column UPDATE item must be a sub-SELECT or
ROW() expression
LINE 1: ...e fvt_obj_operate_update_table_033 set (c_int) = (20) where ...
                                                             ^
postgres=# update fvt_obj_operate_update_table_033 set (c_int,c_dp) =
(20,1.11) where c_int = 20;
UPDATE 1
postgres=#

The cause of this error is because syntax rule conflict between "'(' a_expr
')' opt_indirection" and "implicit_row:    '(' expr_list ',' a_expr ')'".I
think "'(' a_expr ')'" is also a subset of row.
I want to modify it，but didn‘t got a good idea.
Please help,thanks!