[Bug c++/78667] New: nonsensical attribute alloc_size silently accepted

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78667

            Bug ID: 78667
           Summary: nonsensical attribute alloc_size silently accepted
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The documented purpose of the alloc_size attribute is to indicate to the
compiler that a function return value points to region of memory whose size is
given by one or two of the function's integer arguments.

However, GCC accepts even nonsensical specifications of the attribute on
function declarations that do not return pointers, or that do not take any
arguments of integer types.

The program below shows an instance of each of these problems. In the first
declaration, the function returns void and so the attribute is meaningless.  In
the second declaration, the argument associated with the attribute is a pointer
and not an integer.

GCC should reject such non-sensical declarations the same way it rejects
declarations where the argument number referenced by the attribute is out of
bounds.

$ cat a.c && gcc -O2 -S -Wall -Wextra -Wpedantic
-fdump-tree-optimized=/dev/stdout a.c

void f (int) __attribute__ ((alloc_size (1)));
long g (int*) __attribute__ ((alloc_size (1)));

int foo (void)
{
  int i = 17;
  void *p = (void*)g (&i);
  return __builtin_object_size (p, 0);
}

;; Function foo (foo, funcdef_no=0, decl_uid=1799, cgraph_uid=0,
symbol_order=0)

foo ()
{
  int i;

  <bb 2> [100.0%]:
  i = 17;
  g (&i);
  i ={v} {CLOBBER};
  return -1;

}