Funny if only marginally related fact: even though neither the C++11 syntax

  if (int x = foo(); x) { ... }

nor the C++98 syntax

  if (int x = foo()) { ... }

is supported in C99, it still introduces the rule that the entire if statement is a scope (which it wasn’t in C89). So as a party trick, here’s a way to check for C99 without using the preprocessor:

  int c99() {
      enum { C99 = 1 };
      {
          if (sizeof(enum { C99 = 0 })) { }
          return C99;
      }
  }

I make no promises about the behaviour of this code on insufficiently anal compilers like TCC.