C++11 - static_assert within constexpr function?

Question

How would one properly do a static_assert within a constexpr function? For example:

constexpr int do_something(int x)
{
  static_assert(x > 0, "x must be > 0");
  return x + 5;
}

This is not valid C++11 code, because a constexpr function must only contain a return statement. I don't think that the standard has an exception to this, although, the GCC 4.7 does not let me compile this code.

Answer 1

This is not valid C++11 code, because a constexpr function must only contain a return statement.

This is incorrect. static_assert in a constexpr function are fine. What is not fine is using function parameters in constant expressions, like you do it.

You could throw if x <= 0. Calling the function in a context that requires a constant expression will then fail to compile

constexpr int do_something(int x) {   return x > 0 ? (x + 5) : (throw std::logic_error("x must be > 0")); } 

Answer 2

This works and is valid C++11 code, because template arguments are compile time only:

template <int x> constexpr int do_something() {     static_assert(x > 0, "x must be > 0");     return x + 5; } 

I faced with the same problems as you did with constant expressions in C++. There's few clear documentation about constexprs at the moment. And note that there's some known bugs with it in gcc's issue tracker, but your problem seems not to be a bug.

Note that if you declare constexpr functions inside classes, you are not able to use them inside the class. This also seems to be not a bug.

Edit: This is allowed according to the standard: 7.1.3 states

... or a compound-statement that contains only

• null statements,
• static_assert-declarations
• typedef declarations and alias-declarations that do not
  define classes or enumerations,
• using-declarations,
• using-directives,
• and exactly one return statement