Any reason to declare constexpr for a function that returns void?

Question

Here is what I read: using alias for static member functions?

In the answer I see a suggestion to use constexpr. What is the reason to use constexpr for void functions.

Please demonstrate a simple use case. I am new to constexpr so in complex examples I will not understand the gist.

Answer 1

Rahul's answer cites the standard paragraph which allow void constexpr functions, but it doesn't give a use-case. One use-case that comes to my mind would be to have a constexpr class, and as usual factor out behavior common to method in helper methods. The standard explicitly mentions function doing checks, e.g. assertions. I don't have a concrete example at hand, but I can imagine something like

class A
{
public:
    constexpr X doSomething(Y arg1) {
        checkInvariant();
        constraintOnYArgument(arg1);
        // ...
        checkInvariant();        
    }

    constexpr X doSomethingElse(Y arg1) {
        checkInvariant();
        constraintOnYArgument(arg1);
        // ...
        checkInvariant();        
    }

private:
    constexpr void constraintOnYArguments(Y arg) {
    }

    constexpr void checkInvariant() {
        // some checks
        if (!some condition) {
            throw std::logic_error("Oh no!");
        }
    }
};

Answer 2

As per the C++ 14 standard, void is a literal type

A type is a literal type if it is:

— void; or

— a scalar type; or

— a reference type; or

— an array of literal type; or

— a class type (Clause 9) that has all of the following properties: — it has a trivial destructor,

— it is an aggregate type (8.5.1) or has at least one constexpr constructor or constructor template that is not a copy or move constructor, and

— all of its non-static data members and base classes are of non-volatile literal types.

From here:

An arbitrary expression-statement is permitted, in order to allow calls to functions performing checks and to allow assert-like constructs. void also becomes a literal type, so that constexpr functions which exist only to perform such checks may return void.