[Bug c++/107939] New: Rejects use of `extern const` variable in a template

[Bug c++/107939] New: Rejects use of `extern const` variable in a template

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

            Bug ID: 107939
           Summary: Rejects use of `extern const` variable in a template
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
                CC: johelegp at gmail dot com
  Target Milestone: ---

See https://compiler-explorer.com/z/q3a7e7s1e.
Changing `#if 1` to `#if 0` works, so this can't be due to
> The program is ill-formed, no diagnostic required, if:
> (6.4) a hypothetical instantiation of a template immediately following its definition would be ill-formed due to a construct that does not depend on a template parameter, or
> -- https://eel.is/c++draft/temp.res.general#6.4

And since all compilers accept it as a non-template, nothing in [expr.const] is
being violated.

```C++
struct Q {
  struct P {
    const Q* p;
  };
  int n;
  constexpr P operator()(auto) const { return {this}; }
};

extern const Q q;

#if 1
template<int> constexpr auto p = q(0);
static_assert(p<0>.p == &q);
#else
constexpr auto p = q(0);
static_assert(p.p == &q);
#endif

constexpr Q q = {};
```

```
<source>:12:37: error: the value of 'q' is not usable in a constant expression
   12 | template<int> constexpr auto p = q(0);
      |                                     ^
<source>:9:16: note: 'q' was not declared 'constexpr'
    9 | extern const Q q;
      |                ^
<source>:13:22: error: non-constant condition for static assertion
   13 | static_assert(p<0>.p == &q);
      |               ~~~~~~~^~~~~
Compiler returned: 1
```

[Bug c++/107939] [11/12/13 Regression] Rejects use of `extern const` variable in a template

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |needs-bisection
   Target Milestone|---                         |11.4
            Summary|Rejects use of `extern      |[11/12/13 Regression]
                   |const` variable in a        |Rejects use of `extern
                   |template                    |const` variable in a
                   |                            |template
   Last reconfirmed|                            |2022-11-30
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Replacing auto with int (and adding a message to static_assert), gets you a
valid C++14 testcase:
```

struct Q {
  struct P {
    const Q* p;
  };
  int n;
  constexpr P operator()(int) const { return {this}; }
};

extern const Q q;

#if 1
template<int> constexpr auto p = q(0);
static_assert(p<0>.p == &q, "");
#else
constexpr auto p = q(0);
static_assert(p.p == &q, "");
#endif

constexpr Q q = {};
```
This worked with GCC 10.4.0.

[Bug c++/107939] [11/12/13 Regression] Rejects use of `extern const` variable in a template

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

--- Comment #2 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
Making the expression using the `extern const` variable more complex or
indirect makes it work. With an IILE, for example:
https://compiler-explorer.com/z/EjYYvPvqT.

[Bug c++/107939] [11/12/13 Regression] Rejects use of `extern const` variable in a template

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug c++/107939] [11/12/13 Regression] Rejects use of `extern const` variable in a template since r11-557

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |jason at gcc dot gnu.org
           Keywords|needs-bisection             |
            Summary|[11/12/13 Regression]       |[11/12/13 Regression]
                   |Rejects use of `extern      |Rejects use of `extern
                   |const` variable in a        |const` variable in a
                   |template                    |template since r11-557

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This is rejected since r11-557-gbeb019d346b903c16b9fd349937de444b6a8b6c0

[Bug c++/107939] [11/12/13 Regression] Rejects use of `extern const` variable in a template since r11-557

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

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

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

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
A similar example

extern const int z;
template<int> constexpr auto p = z;

works, because require_constant_expression is OK with 'z' here.  The difference
between 'z' here and 'q' in the previous test is
INTEGRAL_OR_ENUMERATION_TYPE_P, which makes decl_maybe_constant_var_p return
different answers:

  else if (CP_TYPE_CONST_NON_VOLATILE_P (type)
           && INTEGRAL_OR_ENUMERATION_TYPE_P (type))
    /* And const integers.  */;

[Bug c++/107939] [11/12/13 Regression] Rejects use of `extern const` variable in a template since r11-557

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I think p_c_e just needs to handle constexpr functors in templates.  I'll poke
more tomorrow.

[Bug c++/107939] [11/12/13 Regression] Rejects use of `extern const` variable in a template since r11-557

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

[Bug c++/107939] [11/12/13 Regression] Rejects use of `extern const` variable in a template since r11-557

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:e09bc034d1b4d692b409fa5af52ae34480a6f4dc

commit r13-6525-ge09bc034d1b4d692b409fa5af52ae34480a6f4dc
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Mar 3 11:24:24 2023 -0500

    c++: error with constexpr operator() [PR107939]

    Similarly to PR107938, this also started with r11-557, whereby
cp_finish_decl
    can call check_initializer even in a template for a constexpr initializer.

    Here we are rejecting

      extern const Q q;

      template<int>
      constexpr auto p = q(0);

    even though q has a constexpr operator().  It's deemed non-const by
    decl_maybe_constant_var_p because even though 'q' is const it is not
    of integral/enum type.

    If fun is not a function pointer, we don't know if we're using it as an
    lvalue or rvalue, so with this patch we pass 'any' for want_rval.  With
    that, p_c_e/VAR_DECL doesn't flat out reject the underlying VAR_DECL.

            PR c++/107939

    gcc/cp/ChangeLog:

            * constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
Pass
            'any' when recursing on a VAR_DECL and not a pointer to function.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/var-templ74.C: Remove dg-error.
            * g++.dg/cpp1y/var-templ77.C: New test.

[Bug c++/107939] [11/12 Regression] Rejects use of `extern const` variable in a template since r11-557

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[11/12/13 Regression]       |[11/12 Regression] Rejects
                   |Rejects use of `extern      |use of `extern const`
                   |const` variable in a        |variable in a template
                   |template since r11-557      |since r11-557

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed on trunk so far.

[Bug c++/107939] [11/12 Regression] Rejects use of `extern const` variable in a template since r11-557

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:19ed6bf44c3fec882cf4c825f3ffa4f2ecdc78e6

commit r12-9232-g19ed6bf44c3fec882cf4c825f3ffa4f2ecdc78e6
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Mar 3 11:24:24 2023 -0500

    c++: error with constexpr operator() [PR107939]

    Similarly to PR107938, this also started with r11-557, whereby
cp_finish_decl
    can call check_initializer even in a template for a constexpr initializer.

    Here we are rejecting

      extern const Q q;

      template<int>
      constexpr auto p = q(0);

    even though q has a constexpr operator().  It's deemed non-const by
    decl_maybe_constant_var_p because even though 'q' is const it is not
    of integral/enum type.

    If fun is not a function pointer, we don't know if we're using it as an
    lvalue or rvalue, so with this patch we pass 'any' for want_rval.  With
    that, p_c_e/VAR_DECL doesn't flat out reject the underlying VAR_DECL.

            PR c++/107939

    gcc/cp/ChangeLog:

            * constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
Pass
            'any' when recursing on a VAR_DECL and not a pointer to function.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/var-templ74.C: Remove dg-error.
            * g++.dg/cpp1y/var-templ77.C: New test.

    (cherry picked from commit e09bc034d1b4d692b409fa5af52ae34480a6f4dc)

[Bug c++/107939] [11 Regression] Rejects use of `extern const` variable in a template since r11-557

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED
            Summary|[11/12 Regression] Rejects  |[11 Regression] Rejects use
                   |use of `extern const`       |of `extern const` variable
                   |variable in a template      |in a template since r11-557
                   |since r11-557               |

--- Comment #9 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.

[Bug c++/107939] [11 Regression] Rejects use of `extern const` variable in a template since r11-557

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:0077c0fb19981c108a01cd15af9b2d6d478c183b

commit r14-5531-g0077c0fb19981c108a01cd15af9b2d6d478c183b
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Nov 16 09:32:07 2023 -0500

    c++: constantness of call to function pointer [PR111703]

    potential_constant_expression for CALL_EXPR tests FUNCTION_POINTER_TYPE_P
    on the callee rather than on the type of the callee, which means we
    always pass want_rval=any when recursing and so may fail to identify a
    non-constant function pointer callee as such.  Fixing this turns out to
    further work around PR111703.

            PR c++/111703
            PR c++/107939

    gcc/cp/ChangeLog:

            * constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
            Fix FUNCTION_POINTER_TYPE_P test.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-fn8.C: Extend test.
            * g++.dg/diagnostic/constexpr4.C: New test.

[Bug c++/107939] [11 Regression] Rejects use of `extern const` variable in a template since r11-557

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:cc4cbf38e842cf023e2bdc63a51ef836d7726d8e

commit r13-8098-gcc4cbf38e842cf023e2bdc63a51ef836d7726d8e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Nov 16 09:32:07 2023 -0500

    c++: constantness of call to function pointer [PR111703]

    potential_constant_expression for CALL_EXPR tests FUNCTION_POINTER_TYPE_P
    on the callee rather than on the type of the callee, which means we
    always pass want_rval=any when recursing and so may fail to identify a
    non-constant function pointer callee as such.  Fixing this turns out to
    further work around PR111703.

            PR c++/111703
            PR c++/107939

    gcc/cp/ChangeLog:

            * constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
            Fix FUNCTION_POINTER_TYPE_P test.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-fn8.C: Extend test.
            * g++.dg/diagnostic/constexpr4.C: New test.

    (cherry picked from commit 0077c0fb19981c108a01cd15af9b2d6d478c183b)

[Bug c++/107939] [11 Regression] Rejects use of `extern const` variable in a template since r11-557

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

--- Comment #12 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:c9cb7e3d75d494a6591887a99d0d3f7f7a307b2e

commit r12-10017-gc9cb7e3d75d494a6591887a99d0d3f7f7a307b2e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Nov 16 09:32:07 2023 -0500

    c++: constantness of call to function pointer [PR111703]

    potential_constant_expression for CALL_EXPR tests FUNCTION_POINTER_TYPE_P
    on the callee rather than on the type of the callee, which means we
    always pass want_rval=any when recursing and so may fail to identify a
    non-constant function pointer callee as such.  Fixing this turns out to
    further work around PR111703.

            PR c++/111703
            PR c++/107939

    gcc/cp/ChangeLog:

            * constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
            Fix FUNCTION_POINTER_TYPE_P test.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-fn8.C: Extend test.
            * g++.dg/diagnostic/constexpr4.C: New test.

    (cherry picked from commit 0077c0fb19981c108a01cd15af9b2d6d478c183b)