[Bug c++/96021] New: missing -Wnonnull passing nullptr to a nonnull variadic lambda

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96021

            Bug ID: 96021
           Summary: missing -Wnonnull passing nullptr to a nonnull
                    variadic lambda
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

G++ issues -Wnonnull when passing nullptr to ordinary variadic functions but
not for the same problem involving variadic lambdas or unspecialized templates.
 I believe the problem is due to the -Wnonull warning checking for arguments of
pointer types and the nullptr constant not having such a type.

$ cat q.C && gcc -O2 -S -Wall q.C
__attribute__ ((nonnull)) void f (int, ...);
void ff ()
{
  f (1, nullptr);    // -Wnonnull (good)
}

template <class T> void g (T t)
{
  t (1, nullptr);    // missing -Wnonnull
}

void gg (void)
{
  g ([](int, auto...) __attribute__ ((nonnull)) { });
}

template <class T> __attribute__ ((nonnull)) void h (T);

void hh ()
{
  h (nullptr);       // missing -Wnonnull
}
q.C: In function ‘void ff()’:
q.C:4:16: warning: argument 2 null where non-null expected [-Wnonnull]
    4 |   f (1, nullptr);    // -Wnonnull (good)
      |                ^
q.C:1:32: note: in a call to function ‘void f(int, ...)’ declared ‘nonnull’
    1 | __attribute__ ((nonnull)) void f (int, ...);
      |                                ^