[Bug c++/94554] New: spurious -Waddress warning within "if constexpr" function-null compares

[Bug c++/94554] New: spurious -Waddress warning within "if constexpr" function-null compares

[myriachan at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

            Bug ID: 94554
           Summary: spurious -Waddress warning within "if constexpr"
                    function-null compares
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: myriachan at gmail dot com
  Target Milestone: ---

The following with -std=c++17 -Waddress:

int meow() { return 1; }
void kitty(int);
template <int (*F)()>
void test() {
    if constexpr (F) {
        kitty(F());
    } else {
        kitty(2);
    }
}
template void test<nullptr>();
template void test<meow>();

gives a spurious/pointless warning:

<source>: In instantiation of 'void test() [with int (* F)() = meow]':
<source>:12:26:   required from here
<source>:5:5: warning: the address of 'int meow()' will never be NULL
[-Waddress]
    5 |     if constexpr (F) {
      |     ^~

The warning should be suppressed in "if constexpr" contexts, because of course
it's going to be always true or always false.

Clang errors on this case, so it's possible that my code is invalid: Is it
legal to compare a function pointer against null in a constant-expression?

[Bug c++/94554] spurious -Waddress warning within "if constexpr" function-null compares

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I don't think this warning should be suppressed in "if constexpr" contexts. 
What's the point of such code?

[Bug c++/94554] spurious -Waddress warning within "if constexpr" function-null compares

[myriachan at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

--- Comment #2 from Melissa <myriachan at gmail dot com> ---
Templates that take an optional function pointer as a template parameter.  It
lets you have templates that change behavior if a null function pointer is
passed.

[Bug c++/94554] spurious -Waddress warning within "if constexpr" function-null compares

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

--- Comment #3 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Melissa from comment #0)
> Clang errors on this case, so it's possible that my code is invalid: Is it
> legal to compare a function pointer against null in a constant-expression?

The example is ill-formed because the condition of 'if constexpr' is more
restricted than that of normal 'if': It expects "a contextually converted
constant expression of type bool" and [expr.const] p10 lists the allowed
conversions in this case. This list omits the boolean conversions
([conv.bool]).

But the example would become valid when rewritten as follows:

int meow() { return 1; }
void kitty(int);
template <int (*F)()>
void test() {
    if constexpr (bool(F)) {
        kitty(F());
    } else {
        kitty(2);
    }
}
template void test<nullptr>();
template void test<meow>();

[Bug c++/94554] spurious -Waddress warning within "if constexpr" function-null compares

[lts-rudolph at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

Klaus Rudolph <lts-rudolph at gmx dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lts-rudolph at gmx dot de

--- Comment #4 from Klaus Rudolph <lts-rudolph at gmx dot de> ---
The code can be modified to be well formed if changed to:

if constexpr (F != nullptr )) {

g++ still emits the bogus warning in that case which is a bug I believe!

[Bug c++/94554] spurious -Waddress warning within "if constexpr" function-null compares

[mail at 3v1n0 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

Marco Trevisan <mail at 3v1n0 dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mail at 3v1n0 dot net

--- Comment #5 from Marco Trevisan <mail at 3v1n0 dot net> ---
I don't think g++ is totally wrong here,

Actually, if any, it should be stronger in making the error clearer and louder,
as per sé it's fine to consider that you're for real not comparing something
constant and the function address may not be known at compile time, so not
wrong to check.

However, something that will not error but work can be something like:

#include <iostream>
#include <type_traits>

using namespace std;

int meow() { return 1; }
void kitty(int i) { std::cout << "Mowed vlaue " << i << "\n"; }

template <int (*F)()>
void test() {
    using NullType = std::integral_constant<decltype(F), nullptr>;
    using ActualType = std::integral_constant<decltype(F), F>;

    if constexpr (!std::is_same_v<ActualType, NullType>) {
        kitty(F());
    } else {
        kitty(222222);
    }
}

int main()
{
    test<nullptr>();
    test<meow>();

    return 0;
}

[Bug c++/94554] spurious -Waddress warning within "if constexpr" function-null compares

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-06-23
             Status|UNCONFIRMED                 |ASSIGNED
                 CC|                            |jason at gcc dot gnu.org

[Bug c++/94554] spurious -Waddress warning within "if constexpr" function-null compares

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

--- Comment #6 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Daniel Krügler from comment #3)
> The example is ill-formed because the condition of 'if constexpr' is more
> restricted than that of normal 'if': It expects "a contextually converted
> constant expression of type bool" and [expr.const] p10 lists the allowed
> conversions in this case. This list omits the boolean conversions
> ([conv.bool]).

This was changed by wg21.link/p1401 so the conversion is allowed in this
context as well.

[Bug c++/94554] spurious -Waddress warning within "if constexpr" function-null compares

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:124a9e08b7a83795bd4d09001955f0eef68ecd00

commit r13-1219-g124a9e08b7a83795bd4d09001955f0eef68ecd00
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Jun 23 00:24:22 2022 -0400

    c++: -Waddress and if constexpr [PR94554]

    Like we avoid various warnings for seemingly tautological expressions when
    substituting a template, we should avoid warning for the implicit
conversion
    to bool in an if statement.  I considered also doing this for the
conditions
    in loop expressions, but that seems unnecessary, as a loop condition is
    unlikely to be a constant.

    The change to finish_if_stmt_cond isn't necessary since dependent_operand_p
    looks through IMPLICIT_CONV_EXPR, but makes it more constent with
    e.g. build_x_binary_op that determines the type of an expression and then
    builds it using the original operands.

            PR c++/94554

    gcc/cp/ChangeLog:

            * pt.cc (dependent_operand_p): Split out from...
            (tsubst_copy_and_build): ...here.
            (tsubst_expr) [IF_STMT]: Use it.
            * semantics.cc (finish_if_stmt_cond): Keep the pre-conversion
            condition in the template tree.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/constexpr-if38.C: New test.

[Bug c++/94554] spurious -Waddress warning within "if constexpr" function-null compares

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94554

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |13.0
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #8 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for GCC 13.