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It would be possible at runtime, yes. Perhaps you can use...
if (sizeof(a) != sizeof(b)) abort();
...type of macro.
It's runtime, but it's better than nothing.
(C++ can do it compile time.)

Indeed. It's potent stuff.

Psuedo code, but...
Something along the lines of:

int main()
{
char mychar;
int myint;
char* c = safe_cast<char>(mychar); // Compiles
c = safe_cast<char>(myint); // Won't...

I don't think this sort of thing can be done in C, or at least not easily.
However, it can be done easily in C++.