The NAME_CONST function is required to work correctly with constants only.
When executed with functions that return types other than those returned by
Item::field_type (string, int, decimal, or real), the result gets cast to
one of those types. This cannot happen for constants.
Fixed by only allowing constants as arguments to NAME_CONST.


mysql-test/r/func_misc.result:
  Bug#30832: Test result
mysql-test/t/func_misc.test:
  Bug#30832: Test case
sql/item.h:
  Bug#30832: The fix