[Bug c++/84194] New: fails to pack structs with template members

[stsp at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84194

            Bug ID: 84194
           Summary: fails to pack structs with template members
           Product: gcc
           Version: 7.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stsp at users dot sourceforge.net
  Target Milestone: ---

Created attachment 43333
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43333&action=edit
test-case

Hello.

Attached is a fully automated test-case
for the problem. Just run "make" and it
will compile test-case with clang and with
gcc, and will compare the output:

---
$ LC_ALL=C make 
clang++ -std=c++11 -o p_clng pod.cpp
g++ -std=c++11 -o p_gcc pod.cpp
pod.cpp:16:23: warning: ignoring packed attribute because of unpacked non-POD
field 'SymWrp<dhdr> lol::_n'
   SymWrp<struct dhdr> _n;
                       ^~
./tst.sh
Test FAILED (sizeof returns 6 instead of 4)
---

This is somewhat similar to bug #83732, but
in #83732 it was considered a diagnostic problem.
Unfortunately it actually produces the bad code,
which is what the test-case demonstrates.

The test code is:

---
#include <stdio.h>
#include <stdint.h>

template<typename T>
class SymWrp : public T {
public:
    SymWrp() = default;
};

struct dhdr {
  uint16_t dh_attr;
};

struct lol {
  unsigned char _l;
  SymWrp<struct dhdr> _n;
  unsigned char _nj;
} __attribute__((packed));

int main()
{
    printf("%zi\n", sizeof(struct lol));
    return 0;
}
---