[Bug c++/84502] [8 Regression] Argument corruption when passing empty templated struct

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84502

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |ABI
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-02-21
      Known to work|                            |7.3.0
            Summary|Argument corruption when    |[8 Regression] Argument
                   |passing empty templated     |corruption when passing
                   |struct                      |empty templated struct
     Ever confirmed|0                           |1
      Known to fail|                            |8.0.1

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is caused by -fabi-version=12 (which is the default for GCC 8). You can
get the old handling of empty structs with -fabi-version=11

The bug only seems to happen when the empty struct is a class template.

Reduced, this fails at -O0 but not any other optimization level, or with
-fabi-version=11:

template<typename T>
struct EmptyTemplatedStruct { };

using X = EmptyTemplatedStruct<int>;

void func(X, int a1, int a2, int a3, int a4, int a5, int a6, int a7, int a8)
{
  if (a1 != 0)
    __builtin_abort();
  if (a2 != 1)
    __builtin_abort();
  if (a3 != 2)
    __builtin_abort();
  if (a4 != 3)
    __builtin_abort();
  if (a5 != 4)
    __builtin_abort();
  if (a6 != 5)
    __builtin_abort();
  if (a7 != 6)
    __builtin_abort();
  if (a8 != 7)
    __builtin_abort();
}

int main()
{
  func(X{}, 0, 1, 2, 3, 4, 5, 6, 7);
}