Revert "Parser: no implicit NOT NULL for system fields [fixes #163]"

This reverts commit 1cfdff5fe3.

Fix it differently.
Cleanup, test results didn't change.

sql/field.cc

@@ -10704,7 +10704,6 @@ Column_definition::Column_definition(THD *thd, Field *old_field,
   pack_flag= 0;
   compression_method_ptr= 0;
   versioning= VERSIONING_NOT_SET;
-  implicit_not_null= false;
 
   if (orig_field)
   {

sql/field.h

@@ -4155,7 +4155,6 @@ public:
     *check_constraint;               // Check constraint
 
   enum_column_versioning versioning;
-  bool implicit_not_null;
 
   Column_definition()
    :Type_handler_hybrid_field_type(&type_handler_null),
@@ -4167,8 +4166,7 @@ public:
     srid(0), geom_type(Field::GEOM_GEOMETRY),
     option_list(NULL), pack_flag(0),
     vcol_info(0), default_value(0), check_constraint(0),
-    versioning(VERSIONING_NOT_SET),
-    implicit_not_null(false)
+    versioning(VERSIONING_NOT_SET)
   {
     interval_list.empty();
   }

sql/sql_show.cc

@@ -2169,10 +2169,9 @@ int show_create_table(THD *thd, TABLE_LIST *table_list, String *packet,
       {
         packet->append(STRING_WITH_LEN(" GENERATED ALWAYS AS ROW END"));
       }
-
-      if (flags & NOT_NULL_FLAG)
+      else if (flags & NOT_NULL_FLAG)
         packet->append(STRING_WITH_LEN(" NOT NULL"));
-      else if (field->type() == MYSQL_TYPE_TIMESTAMP && !field->vers_sys_field())
+      else if (field->type() == MYSQL_TYPE_TIMESTAMP)
       {
         /*
           TIMESTAMP field require explicit NULL flag, because unlike

sql/sql_yacc.yy

@@ -6393,11 +6393,6 @@ field_def:
             }
             DBUG_ASSERT(p);
             *p= field_name;
-            if (lex->last_field->implicit_not_null)
-            {
-              lex->last_field->flags&= ~NOT_NULL_FLAG;
-              lex->last_field->implicit_not_null= false;
-            }
           }
         ;
 
@@ -6636,10 +6631,7 @@ field_type_temporal:
                 Unless --explicit-defaults-for-timestamp is given.
               */
               if (!opt_explicit_defaults_for_timestamp)
-              {
                 Lex->last_field->flags|= NOT_NULL_FLAG;
-                Lex->last_field->implicit_not_null= true;
-              }
               $$.set(opt_mysql56_temporal_format ?
                      static_cast<const Type_handler*>(&type_handler_timestamp2):
                      static_cast<const Type_handler*>(&type_handler_timestamp),
@@ -6830,11 +6822,7 @@ opt_attribute_list:
         ;
 
 attribute:
-          NULL_SYM
-          {
-            Lex->last_field->flags&= ~ NOT_NULL_FLAG;
-            Lex->last_field->implicit_not_null= false;
-          }
+          NULL_SYM { Lex->last_field->flags&= ~ NOT_NULL_FLAG; }
         | DEFAULT column_default_expr { Lex->last_field->default_value= $2; }
         | ON UPDATE_SYM NOW_SYM opt_default_time_precision
           {
@@ -6874,7 +6862,6 @@ asrow_attribute:
           not NULL_SYM
           {
             Lex->last_field->flags|= NOT_NULL_FLAG;
-            Lex->last_field->implicit_not_null= false;
           }
         | opt_primary KEY_SYM
           {