> The short version is, that just isn't possible. Too many things would be unknown, and it is easy in both C and C++ to, at compile time, check the sizeof(size_t) and perform totally different behaviour depending on the value.

I can see an issue with template instantiation in C++: for example, if a template uses SFINAE to specialize a template for types of certain sizes, that needs to be evaluated purely at the C++->LLVM stage of compilation.

What is the compile time part of C do you mean? sizeof(T) is evaluated at compile time of course, but it would still produce pseudocode like:

    if (4 < 16) { // sizeof(T) replaced with 4
        // do something
    } else {
        // do something else
    }

Of course, an optimizer would likely constant-fold that conditional expression to remove the branch entirely, but I'm having a difficult time seeing how one could perform different behavior at compile time with sizeof(T) in C.