[Bug c/85997] New: Bogus -Wvla warning from function array argument with size expression

["kari.nurmela at iki dot fi" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85997

            Bug ID: 85997
           Summary: Bogus -Wvla warning from function array argument with
                    size expression
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kari.nurmela at iki dot fi
  Target Milestone: ---

If there is an array argument in a function definition or declaration that uses
a size expression (C99 6.7.5.3#7, Example 4, Example 5), gcc emits an "ISO C90
forbids variable length array" warning with -Wvla, even when there are no
variable length arrays involved. For example the function declaration

  void print_array(unsigned count, int array[static count]);

gives "not_vla.c:3:1: warning: ISO C90 forbids variable length array "array"
[-Wvla]"

The "array" argument is not a variable-length array, the declaration just
states that the argument provides access to the first element of an array that
has at least "count" elements.

In a whole working example below two warnings are emitted when compiled with
-Wvla (function declaration, line 3, and definition, line 5):


#include <stdio.h>

void print_array(unsigned count, int array[static count]);

void print_array(unsigned count, int array[static count])
{
  for (unsigned i = 0; i < count; ++i)
    {
      printf("%d ", array[i]);
    }
  printf("\n");
}

int main(void)
{
  int array[5] = {1, 2, 3, 4, 5};

  print_array(5, array);

  // print_array(6, array);
  //   Error: array overflow. Since 6 and sizeof array are compile time const,
  //   this could emit a warning, but doesn't.

  return 0;
}