constexpr question

[Rein Halbersma]

But this wasn't the OP's question. Simply initializing a constexpr variable inside foo() and then returning that, will simplify the program at -O0 already. https://gcc.godbolt.org/z/d8YzshsTW

For the macro-lovers, it's possible to define an immediately invoked lambda:

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#define FORCE_CONSTEXPR(expr) [&]() \
    { constexpr auto x = (expr); return x; }()

and write

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if (x & FORCE_CONSTEXPR((Rank6 | Rank7 | Rank8)))

[Sesse]

But this wasn't the OP's question.

No, but neither was OP asking about the C++ standard. When people start talking about “the Standard guarantees” this or that, don't be surprised if those guarantees don't hold.

In practice, every sane compiler in optimizing mode (you don't care about performance on -O0, right?) will constant-fold all the simple cases, constexpr or not.