How Can We Determine if a C 11 Expression is a `constexpr` at Compile Time?

Determining Constexpr Expressions in C 11

Question: Is it possible to determine whether a C 11 expression is a constant expression (constexpr) at compile-time?

Answer: Yes, it is possible using the following technique:

template<typename T>
constexpr typename remove_reference<T>::type makeprval(T&& t) {
  return t;
}

#define isprvalconstexpr(e) noexcept(makeprval(e))

Explanation:

The isprvalconstexpr macro takes an expression e and returns true if e is a prvalue constant expression (i.e., it can be evaluated at compile-time). This is determined by using the noexcept operator, which returns false if e contains any potentially evaluated function calls that do not have a non-throwing exception specification, throw expressions, or throwable dynamic casts or typeids.

Limitations:

It is important to note that this technique has a limitation. The noexcept operator can sometimes give false negatives, meaning it may report that an expression is not a constant expression even though it is. This occurs when the "potentially evaluated" expressions within e are not actually evaluated, which can happen in certain circumstances even though they are formally considered potentially evaluated.

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