The columns that are part of DEFAULT expression were not read-marked
in statements like UPDATE...SET b=DEFAULT.

The problem is `F(DEFAULT)` expression depends of the left-hand side of an
assignment. However, setup_fields accepts only right-hand side value.
Neither Item::fix_fields does.

Suchwise, b=DEFAULT(b) works fine, because Item_default_field has
information on what field it is default of:
    if (thd->mark_used_columns != MARK_COLUMNS_NONE)
      def_field->default_value->expr->update_used_tables();

in Item_default_value::fix_fields().

It is not reasonable to pass a left-hand side to Item:fix_fields, because
the case is rare, so the rewrite
  b= F(DEFAULT)  ->  b= F(DEFAULT(b))

is made instead.

Both UPDATE and multi-UPDATE are affected, however any form of INSERT
is not: it marks all the fields in DEFAULT expressions for read in
TABLE::mark_default_fields_for_write().