MDEV-21995 Server crashes in Item_field::real_type_handler with table value constructor

1. Code simplification:

Item_default_value handled all these values:
a. DEFAULT(field)
b. DEFAULT
c. IGNORE
and had various conditions to distinguish (a) from (b) and from (c).

Introducing a new abstract class Item_contextually_typed_value_specification,
to handle (b) and (c), so the hierarchy now looks as follows:

Item
  Item_result_field
    Item_ident
      Item_field
        Item_default_value                      - DEFAULT(field)
  Item_contextually_typed_value_specification
    Item_default_specification                  - DEFAULT
    Item_ignore_specification                   - IGNORE

2. Introducing a new virtual method is_evaluable_expression() to
determine if an Item is:
- a normal expression, so its val_xxx()/get_date() methods can be called
- or a just an expression substitute, whose value methods cannot be called.

3. Disallowing Items that are not evalualble expressions in table value
   constructors.

1 files changed, 2 insertions, 1 deletions

diff --git a/sql/sql_tvc.cc b/sql/sql_tvc.cc
index 92cd229eebb..def519035d9 100644
--- a/sql/sql_tvc.cc
+++ b/sql/sql_tvc.cc
@@ -59,7 +59,8 @@ bool fix_fields_for_tvc(THD *thd, List_iterator_fast<List_item> &li)
         while replacing their values to NAME_CONST()s.
         So fix only those that have not been.
       */
-      if (item->fix_fields_if_needed(thd, 0))
+      if (item->fix_fields_if_needed(thd, 0) ||
+          item->check_is_evaluable_expression_or_error())
 	DBUG_RETURN(true);
     }
   }