Simple subselects are pulled into upper selects. This operation substitutes the
pulled subselect for the first item from the select list of the subselect.
If an alias is defined for a subselect it is inherited by the replacement item.
As this is done after fix_fields phase this alias isn't showed if the
replacement item is a stored function. This happens because the Item_func_sp::make_field
function makes send field from its result_field and ignores the defined alias.

Now when an alias is defined the Item_func_sp::make_field function sets it for
the returned field.