Macros cannot expand to preprocessing directives. This does not result in a #error-directive, this just results in a syntax error because of an unexpected # token. A better definition would be something that highlights that message in the error output, such as static_assert(0, msg).

That doesn't sound like it would scale at all for any more than a couple of devs.

Neither do TODO comments. This is just for a short-term "don't forget this piece before calling the current job done".

For anything more long-term you want to use a proper task management system with priorities, deadlines, dependencies and so on.

Slight tweak:

    #if DEBUG
    #define TODO(msg) #warning msg
    #else
    #define TODO(msg) #error msg
    #endif

Nice! Isn't this exactly assert in C++? (the following is from https://en.cppreference.com/w/cpp/error/assert)

    #ifdef NDEBUG
    #define assert(condition) ((void)0)
    #else
    #define assert(condition) /*implementation defined*/
    #endif

    ...
    assert(("There are five lights", 2 + 2 == 5));
    ...

    test: test.cc:10: int main(): Assertion `((void)"There are five lights", 2+2==5)' failed.

Take it as a token that you are doing something right. :)

Assert's run-time. This is compile-time (i.e. all instances are guaranteed to trigger during build). C++ equivalent would be a static_assert, but it has a totally different use case like this:

  static_assert(sizeof(StructUsedInSomeBinaryProtocol) == SomeConstantExpectedByTheOtherSide, "<...>")

We used to do static asserts with a preprocessor macro that typedefed an array with size expressed as: (your condition) ? 1 : -1. Point being, static asserts have the use case you want them to have; I see no reason to not have a TODO macro that expands to static_assert(false, "...").