The C core and the C++23 tooling meet at include/astrolune. Ten rules govern what may appear in a header both languages compile, and until now they were followed by review — which means they were followed until someone forgot.
One translation unit that includes every public header and asserts the layout contract turns each of those rules into a compile error. It was the cheapest correctness win left in the tree, and writing it surfaced two real divergences that a human review had walked past twice.
c
/* al_bool is not bool: a real bool collapses 2 to 1 */
static_assert(!std::is_same<al_bool, bool>::value);
static_assert(AL_CAST(al_bool, 2) == 2);
The check is four small files rather than one, and the reasoning is in the boundary document: each file fails in a different way when a specific rule is broken, and collapsing them would blur which rule caught the mistake.