On Wed, 7 Oct 2026, Alejandro Colomar wrote:
> The strict rules would be that it's a constraint to have the forward
> declaration of a parameter contradict the declaration, and thus a
> constraint violation. However, implementing those diagnostics would be
> non-trivial for me.
My suggestion would be to be strict on the new feature (e.g. disallow
_Countof for any parameter for which both a forward declaration and
another declaration of the parameter are visible) rather than adding extra
checks for consistency of forward parameter declarations.
> int
> g(int n, int m, int a[][n]; int n, int m, int a[][m])
> {
> return _Countof(a[0]);
The cases of concern to me are where the length of the parameter array
itself might vary between the declarations (or one declaration might be a
pointer and another an array), because those are the ones where the new
extension of _Countof on array parameters actually comes into play and
starts interacting with forward declarations.
This example is using _Countof on something that has array type in the
existing language for all purposes - not on an array parameter that's
adjusted to a pointer for most purposes. So it already has undefined
behavior at runtime if the two declarations (adjusted to pointer-to-array
type) specify different lengths n and m, and that doesn't depend on the
use of _Countof, only on the function being called with n != m.
--
Joseph S. Myers
[email protected]
