Warning C4834 occurs in the line 355 of concept_check.hpp

[Cheney-W]

I'm a member of Mrcrosoft VCPKG team, when we test vcpkg with an internal version of VS, pagmo2 installation failed with below error:
D:\installed\x64-windows\include\boost/concept_check.hpp(355): error C2220: the following warning is treated as an error
D:\installed\x64-windows\include\boost/concept_check.hpp(355): warning C4834: discarding return value of function with 'nodiscard' attribute

This warning caused by the change: microsoft/STL#1474 , [nodiscard] was added before some operator() function and the change of STL is by design.

Since the warning was reported in the concept_check.hpp file, I think we can make some modifications on boost-concept-check to fix this issue, such as comment the line 355. I'm not sure if this will change the function function, please help to confirm.
Thank you!

[mclow]

What if we just changed line #355 to (void) f(first,second); or (always my favorite) std::ignore = f(first, second).
I'm not really serious about std::ignore, but I like that you can assign anything to it, and it does nothing.

[Cheney-W]

@mclow
Thanks for your comment. I have tested the two modifications you provided and they can all solve the warning C4834.
But as @StarGate-One said, there are 20 downstream ports that depend on the function of concept_check, so could you please help confirm that any of the above two modifications did not change the functionality of library concept_check? Thanks in advance!

[marstaik]

Recently also ran into this issue due to the visual studio 16.9.0 updated.
std::equal_to was tagged with a [[nodiscard]] attribute in the latest stl update.

Confirmed working fix by just casting #355 to (void).

I would recommend a solution like it was done to ignore unused variable warnings in concept_checks.hpp.

1. Create function below ignore_unused_variable_warning line 53 : template<class T> inline void ignore_nodiscard_warning(T const&){}
2. Replace f(arg); in line 326 with ignore_nodiscard_warning(f(arg));
3. Replace f(first, second); in line 355 with ignore_nodiscard_warning(f(first, second));

Remark: I am new in github and still learning the possibilities. I am currently not aware how to create a pull-request with this fix. So I described the changes in the commend.

[mloskot]

I've just asked on the ML [concept_check] Fix issue #31 for Boost 1.76?

[mloskot]

@mclow I confirm your 85fd1e4 fixes the issue. Thanks!

For records, here is a sample to reproduce the issue, https://godbolt.org/z/3Tx341

#include <boost/concept_check.hpp>
#include <functional>
int main()
{
    boost::BinaryFunctionConcept<std::less<int>, bool, int, int>();
}

[gjasny]

@mclow This fix is not on master, yet. Is it too late for 1.76?

[mloskot]

@gjasny Boost 1.76 does ship the 85fd1e4 fix
https://github.com/boostorg/concept_check/blob/boost-1.76.0/include/boost/concept_check.hpp#L355

@mclow I believe this can be closed now.

[Cheney-W]

Thank you for your reply and help.