[Bug c++/104282] New: Copy elision when initializing a base-class subobject with aggregate initialization

[Bug c++/104282] New: Copy elision when initializing a base-class subobject with aggregate initialization

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104282

            Bug ID: 104282
           Summary: Copy elision when initializing a base-class subobject
                    with aggregate initialization
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

The program
```
#include <iostream>

struct A {
  A() { std::cout << "A "; } 
  A(const A&) { std::cout << "Acopy "; } 
  A(A&&) { std::cout << "Amove "; } 
  ~A() { std::cout << "~A "; }
};

struct B : A { };

int main() {
    B b{ A{} };
}
```
if compiled by GCC, prints `A ~A `, showing that there is no copy/move of the
temporary `A{}`.
Clang prints on the other hand `A Amove ~A ~A `, which looks correct, because
of "no elision when initializing a base-class subobject", see
https://en.cppreference.com/w/cpp/language/copy_elision

Demo: https://gcc.godbolt.org/z/rafEsTdd9

[Bug c++/104282] Copy elision when initializing a base-class subobject with aggregate initialization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104282

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Gcc 12 defaults to c++17. I have not looked but I suspect that is the
difference.

[Bug c++/104282] Copy elision when initializing a base-class subobject with aggregate initialization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104282

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
That is c++17 requires Copy elision here while c++14 does not.

[Bug c++/104282] Copy elision when initializing a base-class subobject with aggregate initialization

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104282

--- Comment #3 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
Both Clang and GCC do not change their output either with `-std=c++20` or with
`-std=c++17` options. And both reject the program with -std=c++14`.

[Bug c++/104282] Copy elision when initializing a base-class subobject with aggregate initialization

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104282

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think there's any existing bug about this, and it's an open issue in the ABI

[Bug c++/104282] Copy elision when initializing a base-class subobject with aggregate initialization

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104282

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Related to https://github.com/itanium-cxx-abi/cxx-abi/issues/107

[Bug c++/104282] Copy elision when initializing a base-class subobject with aggregate initialization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104282

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2403

[Bug c++/104282] Copy elision when initializing a base-class subobject with aggregate initialization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104282

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #4)
> I think there's any existing bug about this, and it's an open issue in the
> ABI

PR 98995 .

[Bug c++/104282] Copy elision when initializing a base-class subobject with aggregate initialization

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104282

--- Comment #8 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
One more inconsistency here:

struct A {
  A() {} 
  A(A&&) = delete;
};

struct B : A { };

int main() {
    // ok in GCC
    B{ A{} };

    // error in GCC
    B b{ A{} };
}

GCC allows temporary creation B{ A{} }, but prohibits variable creation B b{
A{} }. Online demo: https://gcc.godbolt.org/z/YGjYYqo6M

It looks like both must be prohibited as in Clang (or both allowed as in MSVC).

Related discussion: https://stackoverflow.com/q/70898525/7325599