[Bug tree-optimization/84202] New: missing -Wnonnull on a returns_nonnull function returning null

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84202

            Bug ID: 84202
           Summary: missing -Wnonnull on a returns_nonnull function
                    returning null
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC silently accepts definitions of functions such as f() below declared with
attribute returns_nonnull.  Clang issues -Wnonnull on functions that trivially
violate that guarantee.

void* __attribute__ ((returns_nonnull)) f ()
{
  return 0;   // missing -Wnonnull
}

Diagnosing trivial mistakes like the one above is only of very limited
usefulness.  GCC can do much better by also diagnosing functions that might
return null such as the two below.  Clang doesn't diagnose these.

void* __attribute__ ((returns_nonnull)) g (unsigned n)
{
  return __builtin_malloc (n);   // missing -Wnonnull
}

void* __attribute__ ((returns_nonnull)) h (unsigned n)
{
  static char buf[256];

  return n < sizeof buf ? buf : __builtin_malloc (n);   // missing -Wnonnull
}