[llvm-bugs] [Bug 37038] New: Trivial copy assignment operator overwrites members not being copied

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37038

            Bug ID: 37038
           Summary: Trivial copy assignment operator overwrites members
                    not being copied
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Keywords: miscompilation
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: hstong at ca.ibm.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

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:
clang++ -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 (clang++ -v):
clang version 7.0.0 (https://github.com/llvm-mirror/clang.git
f38834898ad991aec557e2a609c6b7c3b288bf10)
(https://github.com/llvm-mirror/llvm.git
433eb70569ee6379cdd5181e72b9193fdb0a39dd)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/wandbox/clang-head/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.0.0
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
Candidate multilib: .;@m64
Selected multilib: .;@m64

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