[Bug c++/109470] unexpected const & behavior

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109470

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
You can use -fsanitize=address to get such bugs diagnosed at runtime:
g++ -fsanitize=address -o /tmp/pr109470{,.C} -g; /tmp/pr109470
=================================================================
==2466554==ERROR: AddressSanitizer: stack-use-after-scope on address
0x7ffe08313d80 at pc 0x000000401304 bp 0x7ffe08313d20 sp 0x7ffe08313d18
READ of size 4 at 0x7ffe08313d80 thread T0
    #0 0x401303 in main /tmp/pr109470.C:17
    #1 0x7f5b06f7958f in __libc_start_call_main (/lib64/libc.so.6+0x2958f)
    #2 0x7f5b06f79648 in __libc_start_main@GLIBC_2.2.5
(/lib64/libc.so.6+0x29648)
    #3 0x4010e4 in _start (/tmp/pr109470+0x4010e4)

Address 0x7ffe08313d80 is located in stack of thread T0 at offset 64 in frame
    #0 0x4011b5 in main /tmp/pr109470.C:12

  This frame has 2 object(s):
    [48, 52) 'MAX' (line 14)
    [64, 68) '<unknown>' <== Memory access at offset 64 is inside this variable
HINT: this may be a false positive if your program uses some custom stack
unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-use-after-scope /tmp/pr109470.C:17 in main
Shadow bytes around the buggy address:
  0x10004105a760: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004105a770: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004105a780: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004105a790: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004105a7a0: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 f1 f1 04 f2
=>0x10004105a7b0:[f8]f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004105a7c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004105a7d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004105a7e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004105a7f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10004105a800: 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
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==2466554==ABORTING

C++ has always behaved like this and changing it would significantly penalize
all the code in the wild that uses C++ correctly.  There are some exceptions
where the lifetime is extended, but is certainly not one of them.