flatbuffers.h: fix C++11 compilation (#8857)

[greenrobot]

This constexpr officially works only with C++14 (assignment to lhs, 2 statements) and actually broke some C++11 compilers

Fixes #8857

[jtdavis777]

would it not be better to rewrite these expressions to be constexpr 11 compatible?

inline FLATBUFFERS_CONSTEXPR_CPP11 E operator | (E lhs, E rhs) {
    return E(T(lhs) | T(rhs));
}

inline FLATBUFFERS_CONSTEXPR_CPP11 E operator & (E lhs, E rhs) {
    return E(T(lhs) & T(rhs));
}

inline FLATBUFFERS_CONSTEXPR_CPP11 E operator ^ (E lhs, E rhs) {
    return E(T(lhs) ^ T(rhs));
}

inline FLATBUFFERS_CONSTEXPR_CPP11 E operator ~ (E lhs) {
    return E(~T(lhs));
}

inline FLATBUFFERS_CONSTEXPR_CPP11 bool operator !(E rhs) {
    return !bool(T(rhs));
}

(just ran it through chatgpt, happy to be corrected if necessary :D )

[greenrobot]

would it not be better to rewrite these expressions to be constexpr 11 compatible?

I don't think it can be done, as constexpr cannot contain an assignment (lhs is assigned).

would it not be better to rewrite these expressions to be constexpr 11 compatible?

I doubled checked this (again), ChatGPT (5.2 Thinking) says its not possible.

And lastly, the non-assignment operators (the one you got from ChatGPT) are already present, just above the assignment operators. So, this does not help.

[jtdavis777]

Got it! Sounds good. Figured it was worth a quick check.

[dbaileychess]

C++11 is very old. Do people still target that?

[jtdavis777]

I believe it is still the officially supported version of flatc, and this change doesn't hurt anything to go in

[dbaileychess]

why only these and not the other operators?

If they should not all be changed, the lets leave a comment.

[jtdavis777]

I believe the ones that do not perform assignment can be constexpr in c++11