[sltkr]

There is no need for the second phase. You can record the implicit declaration when you encounter it, and if there is a subsequent declaration, you can check that it's consistent or error out immediately.

This is what C compilers already do, in fact, to produce warnings when an implicit declaration doesn't match a later explicit declaration. But this is a best-effort warning only; it doesn't work if there is no declaration because the function is defined in a different translation unit, as I pointed out above.