with clang, [[nodiscard]] is emitted also for -std=c++11

[botovq]

What version of protobuf and what language are you using?
Version: v3.19.1
Language: C++

What operating system (Linux, Windows, ...) and version?

OpenBSD -current

What runtime / compiler are you using (e.g., python version or gcc version)

clang 11.1.0

What did you do?

Build protobuf 3.19.1, then try to build protozero 1.7.0.

What did you expect to see

protozero 1.7.0 builds

What did you see instead?

The build failed because protozero builds with -pedantic -std=c++11 -Werror. On the one hand, this is enough to have PROTOBUF_NODISCARD defined as [[nodiscard]]. On the other hand, this then trips over -Werror due to -Wc++17-extensions:

[79/90] /tmp/pobj/protozero-1.7.0/bin/c++  -I/tmp/pobj/protozero-1.7.0/protozero-1.7.0/include -I/tmp/pobj/protozero-1.7.0/protozero-1.7.0/test/include -isystem /tmp/pobj/protozero-1.7.0/protozero-1.7.0/test/catch -isystem /usr/local/include -isystem test -O2 -pipe  -std=c++17 -DNDEBUG   -std=c++11 -Wall -Wextra -pedantic -Wsign-compare -Wunused-parameter -Wno-float-equal -Wno-covered-switch-default -Werror -pthread -MD -MT test/CMakeFiles/writer_tests.dir/t/bytes/bytes_testcase.pb.cc.o -MF test/CMakeFiles/writer_tests.dir/t/bytes/bytes_testcase.pb.cc.o.d -o test/CMakeFiles/writer_tests.dir/t/bytes/bytes_testcase.pb.cc.o -c test/t/bytes/bytes_testcase.pb.cc
FAILED: test/CMakeFiles/writer_tests.dir/t/bytes/bytes_testcase.pb.cc.o /tmp/pobj/protozero-1.7.0/bin/c++  -I/tmp/pobj/protozero-1.7.0/protozero-1.7.0/include -I/tmp/pobj/protozero-1.7.0/protozero-1.7.0/test/include -isystem /tmp/pobj/protozero-1.7.0/protozero-1.7.0/test/catch -isystem /usr/local/include -isystem test -O2 -pipe  -std=c++17 -DNDEBUG   -std=c++11 -Wall -Wextra -pedantic -Wsign-compare -Wunused-parameter -Wno-float-equal -Wno-covered-switch-default -Werror -pthread -MD -MT test/CMakeFiles/writer_tests.dir/t/bytes/bytes_testcase.pb.cc.o -MF test/CMakeFiles/writer_tests.dir/t/bytes/bytes_testcase.pb.cc.o.d -o test/CMakeFiles/writer_tests.dir/t/bytes/bytes_testcase.pb.cc.o -c test/t/bytes/bytes_testcase.pb.cc
In file included from test/t/bytes/bytes_testcase.pb.cc:4:
test/t/bytes/bytes_testcase.pb.h:190:3: error: use of the 'nodiscard' attribute is a C++17 extension [-Werror,-Wc++17-extensions]
  PROTOBUF_NODISCARD std::string* release_s();
  ^
/usr/local/include/google/protobuf/port_def.inc:463:30: note: expanded from macro 'PROTOBUF_NODISCARD'
#define PROTOBUF_NODISCARD [[nodiscard]]
                             ^
1 error generated.
ninja: build stopped: subcommand failed.

Anything else we should know about your project / environment

It seems #if __has_cpp_attribute(nodiscard) is insufficient to determine whether [[nodiscard]] is actually allowed. A similar issue was also reported here: nlohmann/json#1535

This could be avoided if PROTOBUF_NODISCARD were only defined to [[nodiscard]] with

#if __has_cpp_attribute(nodiscard) && PROTOBUF_CPLUSPLUS_MIN(201703L)
#define PROTOBUF_NODISCARD [[nodiscard]]
#elif
...

or similar.

[thomas-olszamowski]

Confirming same problem with:
Apple clang version 13.0.0
and protobuf 3.19.2

[ezzieyguywuf]

To add some more context: it appears that 624d29d added these lines to port_def.inc, which is where the error is originating from.

I think @botovq 's suggestion to extend the logic in the preprocessor check should help to mitigate this.

[ezzieyguywuf]

Here's how you can reproduce the error with a minimal example:

protoc --version # must be >= 3.18.1

# test.proto
syntax = "proto2";

package Nothing;

message Message {
  optional string something = 1;
}

// main.cpp
#include "test.pb.h"

int main(){}

# first, generate the protobuf code
protoc -I=. --cpp_out=. test.proto

# this works
clang++ -std=c++17 -pedantic -Werror main.cpp

# this does not
clang++ -std=c++14 -pedantic -Werror main.cpp