constexpr - why just a return statement?

Question

A constexpr function must just consist of a return statement and every argument should be known at compile time:

// constexpr functions use recursion rather than iteration
constexpr int factorial(int n)
{
    return n <= 1 ? 1 : (n * factorial(n-1));
}

Why just the return statement? I mean, why this is wrong?

// constexpr functions use recursion rather than iteration
constexpr int factorial(int n)
{
    int a = 222; //another variable
    return n <= 1 ? 1 : (n * factorial(n-1));
}

Answer 1

Why:

Because constexpr is a fairly new and radical concept in C++11 and it is difficult to transition a major language standard to something completely new. Conservatism rules.

For C++1y (currently targeted at C++14), your example is legal. Tip of trunk clang already implements it under the -std=c++1y flag.

Answer 2

It simplifies the implementation, as Andy Prowl says. That possibly answers "Why", but it doesn't say how it does that.

A function with only a return value, and more specifically one without local variables, is a special situation for a compiler. This function is now comprised of a single expression: the function's AST needs only to have a single root. That lack of variables means this expression can be evaluated without a full-blown virtual machine to process it, rather a simple tree expression evaluator can be used. For various reasons the compiler probably already has such an evaluator, or could create one relatively easily (it becomes a tree simplification pass).

Knowing that only constexpr is used inside the expression also provides a key simplification. This guarantees that each vertex in the function AST has the same properties, even if it is a function call. The entire constexpr mechanism is then a generalized form of const-folding. And though it isn't always done at this high level in the compiler, it ensures it can be implemented without a huge effort (compared to a full VM).

Back to the "why" question. The restriction is driven primarily by resource limitations on the vendors. This feature, as specified, is not a huge effort and thus the vendors can actually implement it in a reasonable period of time. If there were no such restrictions, in particular allowing local variables, it greatly increases the amount of work needed. From the user's perspective (us, the programmers), the restrictions are however entirely arbitrary.