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

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

[msebor at gcc dot 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.

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

[pinskia at gcc dot gnu.org]

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
GCC warns only at instantiation time.

[Bug c++/102378] missing -Waddress in template code at definition time rather than instantiation

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|missing -Waddress in        |missing -Waddress in
                   |template code               |template code at definition
                   |                            |time rather than
                   |                            |instantiation
           Severity|normal                      |enhancement
           Keywords|                            |diagnostic

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> GCC warns only at instantiation time.

And it has been that way since at least 4.4.0.

[Bug c++/102378] missing -Waddress in template code at definition time rather than instantiation

[msebor at gcc dot gnu.org]

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

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
With the correct explicit instantiation directive things look much better:

$ cat pr102378.C && gcc -S -Wall pr102378.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;    // -Waddress (good)
  }

  {
    T t;
    return &t == 0;    // -Waddress (good)
  }

  {
    T a[2];
    return &a == 0;    // -Waddress (good)
  }
}

template int h<int> ();
pr102378.C: In function ‘int f()’:
pr102378.C:4:13: warning: the address of ‘a’ will never be NULL [-Waddress]
    4 |   return &a == 0;      // -Waddress (good)
      |          ~~~^~~~
pr102378.C: In instantiation of ‘int h() [with T = int]’:
pr102378.C:45:22:   required from here
pr102378.C:31:15: warning: the address of ‘a’ will never be NULL [-Waddress]
   31 |     return &a == 0;    // -Waddress (good)
      |            ~~~^~~~
pr102378.C:36:15: warning: the address of ‘t’ will never be NULL [-Waddress]
   36 |     return &t == 0;    // -Waddress (good)
      |            ~~~^~~~
pr102378.C:41:15: warning: the address of ‘a’ will never be NULL [-Waddress]
   41 |     return &a == 0;    // -Waddress (good)
      |            ~~~^~~~

[Bug c++/102378] missing -Waddress in template code at definition time rather than instantiation

[msebor at gcc dot gnu.org]

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

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2021-09-16
   Target Milestone|---                         |12.0
     Ever confirmed|0                           |1

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> (In reply to Andrew Pinski from comment #1)
> > GCC warns only at instantiation time.
> 
> And it has been that way since at least 4.4.0.

Not quite.  It depends on where a warning is implemented.  As the test case
below shows, G++ does diagnose some problems in uninstantiated templates but
not others.  Jason's suggestion in the review I pointed to is to try to do
better and the purpose of this bug is to capture that limitation and
improvement for the work I'm doing.

$ cat t.C && gcc -S -Wall -Wredundant-tags t.C
struct A { A (); ~A (); };

template <class T>
void f ()
{
  volatile struct A a;            // -Wredundant-tags (good)
  register int n = sizeof a;      // -Wregister (good)
  __builtin_memset (&a, 0, n);    // missing -Wclass-memaccess and volatile A*
to A* conversion warning
}

t.C: In function ‘void f()’:
t.C:6:12: warning: redundant class-key ‘struct’ in reference to ‘struct A’
[-Wredundant-tags]
    6 |   volatile struct A a;            // -Wredundant-tags (good)
      |            ^~~~~~
      |            ------
t.C:7:16: warning: ISO C++17 does not allow ‘register’ storage class specifier
[-Wregister]
    7 |   register int n = sizeof a;      // -Wregister (good)
      |                ^

[Bug c++/102378] missing -Waddress in template code at definition time rather than instantiation

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.0                        |12.2

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 12.1 is being released, retargeting bugs to GCC 12.2.

[Bug c++/102378] missing -Waddress in template code at definition time rather than instantiation

[rguenth at gcc dot gnu.org]

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.3                        |12.4

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 12.3 is being released, retargeting bugs to GCC 12.4.