[Bug c/112761] New: Using incomplete array types in function prototypes doesn't work

["ville.syrjala at linux dot intel.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112761

            Bug ID: 112761
           Summary: Using incomplete array types in function prototypes
                    doesn't work
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ville.syrjala at linux dot intel.com
  Target Milestone: ---

struct foo;

void func(struct foo bar[]);

int main(void)
{
}

error: array type has incomplete element type ‘struct foo’
    3 | void func(struct foo bar[]);
      |                      ^~~

This prevents using the array syntax in function prototypes with opaque types.
So I have to give up either:
- the array syntax, which means there is no longer any hint to the reader
whether the function takes an array or a pointer to a single object
- or the opaque type which is even worse since the clean abstraction is ruined
(not to mention the resulting stable ABI issues).

I couldn't immediately find anything in the spec that says this is illegal:
- "6.7.6.2 Array declarators ...
If the size is not present, the array type is an incomplete type."
- "6.7.6.3 Function declarators ...
If the function declarator is not part of a definition of that function,
parameters may have incomplete type and may use the [*] notation in their
sequences of declarator specifiers to specify variable length array types."

But maybe I just don't know how to read the spec correctly?

Side note:
This also affects the (arguably hideous) "ptr[static 1]" syntax which would be
beneficial to indicate that you can't pass in a null pointer. But I suppose
that one is explicitly forbidden by the spec since we do specify a size for the
"array" and thus it is not allowed to be an incomplete type.