equality == comparison between not_null and normal pointer works with Microsoft 2019 compiler, but not with g++ 11 compiler

[daltairwalter]

The below code compiles on windows with MSVC 2019, it appears to be using the implicit conversion operator and doing the equality comparison as a standard pointer comparison. On linux using g++11, the following code fails to compile and generates an error:

   13 |     if (notNull == test)
      |         ~~~~~~~ ^~ ~~~~
      |         |          |
      |         |          Test*
      |         gsl::not_null<Test*>

I have tried various c++ versions with -std:c++14,17,20 and /permissive- and none of these options seem to make the MS compiler fail or the g++ compiler pass.

The provided equality operator only seems to be effective when both inputs are gsl::not_null - given this state, I am wondering if it would make sense to make the equality operator a class member so that it isn't considered for all of the equality comparisons that don't involve not_null.

#include "gsl/gsl"
#include <iostream>
#include <memory>

class Test
{
};

int main()
{
    auto test = new Test();
    auto notNull = gsl::not_null<Test*>(test);
    if (notNull == test)
    {
        std::cout << "not_null == Test*" << std::endl;
    }
    else
    {
        std::cout << "not_null != Test*" << std::endl;
    }
    return 0;
}

[carsonRadtke]

Could you update the issue to include the complete error message?

I am unable to reproduce the compilation failure on gcc-11 (https://godbolt.org/z/PM6f9fx49). Could you please provide more information:

• Confirm you are using GSL v4.1.0
• Output of $ gcc --version?
• A compiler explorer link

[daltairwalter]

I had changes to templatize the implicit conversion operator see:
#1187

The templatized implicit conversion operator seems to work, but for this equality to work with g++, the non-templatized implicit conversion operator is also needed.

Sorry about the confusion - this is working.