Add dedicated operator== overload between signal and pointer_type

[b1f6c1c4]

C++20 has significant modifications on the overloads of operator==. See the following demo: https://godbolt.org/z/dTvncj

In order to ease the transition to C++20 some day, it'd be better to stop relying on the C++17's operator== behavior; otherwise, the following error would occur when compiling the existing code base using C++20:

../test/networks/xag.cpp:264:19: error: ambiguous overload for ‘operator==’ (operand types are ‘const mockturtle::node_pointer<1>’ and ‘const mockturtle::xag_network::signal’)
  264 |       CHECK( node == f1 ); /* first po */

[b1f6c1c4]

Besides, it would be better to add another item(s) into .github/workflows/*.yml that verifies C++20-compliance of the source code. It may not be necessary to change the global CMakeList.txt to C++20 at this moment, IMO.

[coveralls]

Pull Request Test Coverage Report for Build 599328370

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 79.787%

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Totals
Change from base Build 592722214:	0.008%
Covered Lines:	10496
Relevant Lines:	13155

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💛 - Coveralls

[hriener]

Thank you for the PR!

As you have noticed, we do not support C++20 yet. However, I appreciate the changes to prepare mockturtle for the future.

Do you have a suggestion for the workflow update? What's the compiler that you would like to target?

[b1f6c1c4]

@hriener I believe GCC >=10.2.0 would do the job (I'm personally using 10.2.0 locally.)
Besides, I think passing some parameters to the cmake command line in somewhere like https://github.com/lsils/mockturtle/blob/master/.github/workflows/linux.yml#L24 would be beneficial.

[hriener]

Agreed. I'm happy to merge the workflow/CMake update if you would like to commit the changes to the branch (assuming that the required changes are not too invasive).

[b1f6c1c4]

I'm not very familiar with github workflows so maybe you could merge the PR first and the modify .github/workflows/*.yml later.

[hriener]

I like your idea of adding a c++20 compiler first to verify that the changes are indeed required and correct. Also, updating the workflows should be not too complicated; let me see if I can get it done by the end of the week.

[hriener]

Thanks!