[Bug c++/85271] New: Trivial copy assignment operator overwrites members not being copied

["hstong at ca dot ibm.com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85271

            Bug ID: 85271
           Summary: Trivial copy assignment operator overwrites members
                    not being copied
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hstong at ca dot ibm.com
  Target Milestone: ---

In the following, the operator= used by Q's defaulted copy assignment operator
for the "d" subobject is a copy assignment operator of B: that is, it does not
copy the "b0" member of "d".
The expected behaviour is observed when Q's copy assignment operator is
explicitly defaulted after its first declaration, but not if it is explicitly
defaulted on its first declaration.

### SOURCE (<stdin>):
extern "C" void abort(void);
struct B0 { int b0; };

struct B {
  B &operator=(const B &) = default;
  int x;
};

struct D : B0, B {
  using B::operator=;
private:
  D &operator=(const D &) && = default;
};

struct Q {
  Q &operator=(const Q &) = default;
  D d;
};
//Q &Q::operator=(const Q &) = default;

Q qa, qb;
int main(void) {
  qb.d.b0 = 42;
  qb = qa;
  if (qb.d.b0 != 42) abort();
}


### COMPILER INVOCATION:
g++ -xc++ -std=c++2a -o inhtrivcpyassn -


### INVOCATION AND OUTPUT OF RESULTING EXECUTABLE:
> ./inhtrivcpyassn; printf '(rc=%d)\n' "$?"
Aborted                 ./inhtrivcpyassn
(rc=134)


### EXPECTED RUN OUTPUT:
(rc=0)


### COMPILER VERSION INFO:
Using built-in specs.
COLLECT_GCC=/opt/wandbox/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-head/libexec/gcc/x86_64-pc-linux-gnu/8.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../source/configure --prefix=/opt/wandbox/gcc-head
--enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl
--enable-checking=release --disable-nls --enable-lto
LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-head/lib,-rpath,/opt/wandbox/gcc-head/lib64,-rpath,/opt/wandbox/gcc-head/lib32
Thread model: posix
gcc version 8.0.1 20180405 (experimental) (GCC)