List them. I am not aware of any well defined parts of the C standard where GCC and Clang disagree in implementation. Only in areas where things are too vague (and are effectively either unspecified or undefined), or understandably in areas where they're "implementation defined".
If there are behaviours where a compiler deviates from the standard it is either something you can configure (e.g. -ftrapv or -fwrapv) or it's a bug.
> Also compilers have bugs.
Nothing you do can defend against compiler bugs outside of extensively testing your results. If you determine that a compiler has a bug then the correct course of action is definitely not: "note it down and incorporate the understanding into your future programs"
> So a proficient C/C++ programmer does have to learn what compilers actually do in practice and what they guarantee beyond the standard (or how they differ from it).
There are situations where it's important to know what the compiler is doing. But these situations are limited to performance optimisation, the knowledge gained through these situations should only be applied to the single version of the compiler you observed it in, and you should not use the knowledge to feed back to your understanding of C or the implementation.
It's almost impossible to decipher how modern C compilers work exactly and trying to determine what an implementation does based on the results of compilation is therefore extremely unreliable. If you need to rely on implementation defined behaviour (unavoidable in any real program) then you should be relying solely on documentation, and if the observed behaviour deviates from the documentation then that is, again, a bug bug.
> It isn't, but it is a family of languages that share a lot of syntax and semantics.
I am not a C/C++/C#/ObjectiveC/JavaScript/Java programmer.
C++ and C might share a lot of syntax but that's basically where the similarities end in any modern implementation. People who know C thinking they know enough C to write reliable and conformant C++ and people who know C++ thinking they know enough C++ to write reliable and conformant C are one of the groups of people who produce the most subtle mistakes in these languages.
I think you could get away with these kinds of things in the 80s but that has definitely not been the case for quite a while.