[Bug c++/102378] New: missing -Waddress in template code

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102378

            Bug ID: 102378
           Summary: missing -Waddress in template code
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

While implementing the suggestion in the review below I noticed that G++ fails
to issue -Waddress in template code even when it could:
https://gcc.gnu.org/pipermail/gcc-patches/2021-September/579082.html

$ cat t.C && gcc -O2 -S -Wall -Waddress t.C
int f ()
{
  int a[2];
  return &a == 0;      // -Waddress (good)
}

template <class T>
int g ()
{
  {
    int a[2];
    return &a == 0;    // missing -Waddress
  }

  {
    T t;
    return &t == 0;    // no -Waddress for type-dependent expr (good)
  }

  {
    T a[2];
    return &a == 0;    // missing -Waddress
  }
}

template <class T>
int h ()
{
  {
    int a[2];
    return &a == 0;    // missing -Waddress
  }

  {
    T t;
    return &t == 0;    // missing -Waddress for T = int
  }

  {
    T a[2];
    return &a == 0;    // missing -Waddress
  }
}

template <int> int h ();
t.C: In function ‘int f()’:
t.C:4:13: warning: the address of ‘a’ will never be NULL [-Waddress]
    4 |   return &a == 0;      // -Waddress (good)
      |          ~~~^~~~


In contrast, Clang issues three warnings:

t.C:4:11: warning: comparison of address of 'a' equal to a null pointer is
      always false [-Wtautological-pointer-compare]
  return &a == 0;      // -Waddress (good)
          ^    ~
t.C:12:13: warning: comparison of address of 'a' equal to a null pointer is
      always false [-Wtautological-pointer-compare]
    return &a == 0;    // missing -Waddress
            ^    ~
t.C:31:13: warning: comparison of address of 'a' equal to a null pointer is
      always false [-Wtautological-pointer-compare]
    return &a == 0;    // missing -Waddress
            ^    ~
3 warnings generated.