Strange compile error when g++ work with std=c++20.

[Andrew Pinski]

On Mon, Nov 27, 2023 at 8:24 PM Lew Robin via Gcc-bugs
<gcc-bugs@gcc.gnu.org> wrote:
>
> This error happens when using macro and template.
> GCC Version: gcc version 12.3.0 (Ubuntu 12.3.0-1ubuntu1~22.04)
> OS: ubuntu 22.04 (x64)
> Compile Command:
> g++-12 ./testmacro.cc --std=c++20
>
> In fact, this error exisits from g++11 to g++13.  I also test it on clang and msvc, but it cannot be reproduced.

GCC is correct here.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103593 which points
to C++20 DR 2237 (and the bug about diagnostic that should be improved
is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97202).
I suspect clang and MSVC has not implemented that C++ defect report
which is why it is accepted by those 2.

Thanks,
Andrew Pinski


>
> short reproduction:
> #include <mutex>
>
> #define DECLARE_SINGLETON(classname_type)     \
>   public:                                     \
>   classname_type();
>
> template <class MessageT>
> class ReceiverManager
> {
> public:
>   ~ReceiverManager() { }
>
> private:
>   MessageT receiver_map_;
>   DECLARE_SINGLETON(ReceiverManager<MessageT>)
> };
>
> template <typename MessageT>
> ReceiverManager<MessageT>::ReceiverManager(){}
>
> int main()
> {
>   auto m = ReceiverManager<int>();
>   return 0;
> }
> compile it and report error:
>
> ./testmacro.cc:5:24: error: expected unqualified-id before ‘)’ token
>     5 |         classname_type();
>       |                        ^
> ./testmacro.cc:15:9: note: in expansion of macro ‘DECLARE_SINGLETON’
>    15 |         DECLARE_SINGLETON(ReceiverManager<MessageT>)
>       |         ^~~~~~~~~~~~~~~~~
> ./testmacro.cc:19:1: error: no declaration matches ‘ReceiverManager<MessageT>::ReceiverManager()’
>    19 | ReceiverManager<MessageT>::ReceiverManager(){}
>       | ^~~~~~~~~~~~~~~~~~~~~~~~~
> ./testmacro.cc:19:1: note: no functions named ‘ReceiverManager<MessageT>::ReceiverManager()’
> ./testmacro.cc:8:7: note: ‘class ReceiverManager<MessageT>’ defined here
>     8 | class ReceiverManager
>       |       ^~~~~~~~~~~~~~~