[Bug c/49748] New: char * const * cannot be assigned to char const * const *

[gcc at misc dot lka.org.lu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49748

           Summary: char * const * cannot be assigned to char const *
                    const *
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: g...@misc.lka.org.lu


Trying to assign a pointer value A with type "char * const *" into a variable B
declared as "char const * const * " triggers a warning, although the
const-yness of the type is _increased_ not decreased.

Such a thing may be needed when trying to call a function that displays a list
of strings:

void displayList(char const * const * list)
{
   /* display list */
}

The char const * declaration is needed if somewhere in the program, this
function is called with an array of static strings:

   char const  *list1[] = { "hello", "world", NULL };
   displayList(list1);


However, if elsewhere in the program, we want to call the function with a list
of _dynamically_ allocated strings, we get a "initialization from incompatible
pointer type" warning:
   char **list2;
   for(i=0; i< N; i++)
      list2[i] = strdup( some expression );

   ...
   displayList(list2);

   for(i=0; i<N; i++)
      free(list2[i]);

Because of the free(), we cannot declare list2[] as char const ** , but then
this restriction on _adding_ the const qualifier during assignment or parameter
passing is causing trouble.


There is a very similar case, where such a restriction on adding a const
qualifier makes sense:

  assiging  char **  to  char const **

(i.e. the case where in the target, only the string _contents_ are const, but
not the string pointers themselves).

In this case, the assignment warning would be appropriate, in order to prevent
the following:

char *launderConst(char const *in)
{
    char * source[1];
    char const ** target = source;

    target[0] = in;
    return source[0];
}


However if the target list itself is const (rather than just its string
contents), we would never be able to do this:
    char const *const* target = source;
    target[0] = in;

So why the warning about the first line, if the second one is forbidden anyways
(assigning a value to a const)?


Or is there another way of declaring the displayList() function without
triggering a warning in any of both use-cases?