19. Static assertions

The P4 core library contains two overloaded declarations for a static_assert function, as follows:

/// Static assert evaluates a boolean expression
/// at compilation time.  If the expression evaluates to
/// false, compilation is stopped and the corresponding message is printed.
extern bool static_assert(bool check, string message);

/// Like the above but using a default message.
extern bool static_assert(bool check);

These functions both return boolean values. Since the parameters are directionless, these functions require compile-time known values as arguments, thus they can be used to enforce compile-time invariants. Since P4 does not allow statements at the program top-level (outside of apply blocks), these functions can be used at the top-level by assigning their result to a dummy constant, e.g.:

const bool _check = static_assert(V1MODEL_VERSION > 20180000,
                                  "Expected a v1 model version >= 20180000");

As the comment indicates, if static_assert returns false, it causes the program compilation to be terminated immediately with an error.