[Bug ada/65696] New: ASAN reports global-buffer-overrun for local tagged types

["demoonlit at panathenaia dot halfmoon.jp" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65696

            Bug ID: 65696
           Summary: ASAN reports global-buffer-overrun for local tagged
                    types
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ada
          Assignee: unassigned at gcc dot gnu.org
          Reporter: demoonlit at panathenaia dot halfmoon.jp

package pkg_a is
   type Root is tagged null record;
end pkg_a;

with pkg_a;
procedure main is
   type Derived is new pkg_a.Root with null record;
begin
   null;
end main;

% gnatmake main.adb -cargs -fsanitize=address -largs -lasan.1
gcc -c -fsanitize=address main.adb
gcc -c -fsanitize=address pkg_a.ads
gnatbind -x main.ali
gnatlink main.ali -lasan.1

% ./main 
=================================================================
==78928==ERROR: AddressSanitizer: unknown-crash on address 0x0001000174c8 at pc
0x10000303d bp 0x7fff5fbfed10 sp 0x7fff5fbfed08
READ of size 120 at 0x0001000174c8 thread T0
    #0 0x10000303c (main+0x10000303c)
    #1 0x1000023d4 (main+0x1000023d4)
    #2 0x1000019d7 (main+0x1000019d7)
    #3 0x0 (main+0x0)

0x000100017510 is located 0 bytes to the right of global variable
'pkg_a__rootR38s___UNC' from 'pkg_a.ads' (0x1000174c0) of size 80
SUMMARY: AddressSanitizer: unknown-crash ??:0 ??
Shadow bytes around the buggy address:
  0x100020002e40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100020002e50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100020002e60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100020002e70: 00 00 00 00 00 00 00 00 00 f9 f9 f9 f9 f9 f9 f9
  0x100020002e80: 00 f9 f9 f9 f9 f9 f9 f9 00 00 00 00 00 00 00 00
=>0x100020002e90: 00 00 f9 f9 f9 f9 f9 f9 00[00]00 00 00 00 00 00
  0x100020002ea0: 00 00 f9 f9 f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9
  0x100020002eb0: 00 00 00 00 00 f9 f9 f9 f9 f9 f9 f9 00 00 00 00
  0x100020002ec0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100020002ed0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100020002ee0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Contiguous container OOB:fc
  ASan internal:           fe
==78928==ABORTING

Perhaps, a cause is the implicit code for making a child type's dispatching
table.
The size of copying of a parent type's predefined primitives to a child's is
Max_Predef_Prims (15), but the real number of a parent type's predefined
primitives seems 9.