[Bug c++/97700] New: Bogus error when a class containing a function pointer is used as a non-type template parameter

[Bug c++/97700] New: Bogus error when a class containing a function pointer is used as a non-type template parameter

[ensadc at mailnesia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

            Bug ID: 97700
           Summary: Bogus error when a class containing a function pointer
                    is used as a non-type template parameter
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ensadc at mailnesia dot com
                CC: jason at redhat dot com
  Target Milestone: ---

Originally discovered by cigien on StackOverflow. See
https://stackoverflow.com/questions/64655958/can-a-class-containing-a-function-pointer-be-used-as-a-non-type-template-parameter

GCC does not like this program:

````

struct S 
{ 
    void (*f)(); 
}; 

constexpr S s {[]{}};

template<S> struct X {};
using x = X<s>;

````

It complains with:

````
error: '<lambda()>::_FUN' is not a valid template argument of type 'void (*)()'
because '<lambda()>::_FUN' is not a variable
using x = X<s>;
             ^
````

As mentioned in the StackOverflow post, it appears that
`invalid_tparm_referent_p` (called by `get_template_parm_object`) fails to
account for the pointer-to-function case.

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[ensadc at mailnesia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

ensadc at mailnesia dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=83258

--- Comment #1 from ensadc at mailnesia dot com ---
The lambda is not needed to reproduce the bug. This also triggers the error:

void fun() {}
struct S 
{ 
    void (*f)(); 
}; 

template<S> struct X {};
using x = X<S{ fun }>;

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-11-19
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

--- Comment #3 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
This seems to be a duplicate of Bug 83258.

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |waffl3x at protonmail dot com

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 109168 has been marked as a duplicate of this bug. ***

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[mail at lboeger dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

--- Comment #5 from Lukas Böger <mail at lboeger dot de> ---
One part of this seems to be resolved with version 13. Below, only the lambda
use case fails to compile, whereas prior to version 13, both type aliases are
rejected:

```
void fun() {}
struct S 
{ 
    void (*f)(); 
}; 

template<S> struct X {};

using works = X<S{ fun }>;
using does_not_work = X<S{ []{}}>;
```

See https://godbolt.org/z/3E7WE6zKo

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

Patrick Palka <ppalka at gcc dot gnu.org> changed:

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

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Partially fixed by r13-6970-gb5e38b1c166357

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

--- Comment #7 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:c3afdb8ba8f1839544c414f57e41a58c8fda5349

commit r14-708-gc3afdb8ba8f1839544c414f57e41a58c8fda5349
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu May 11 10:04:25 2023 -0400

    c++: converted lambda as template argument [PR83258, ...]

    r8-1253-g3d2e25a240c711 removed the template argument linkage requirement
    in convert_nontype_argument for C++17 (which r9-3836-g4be5c72cf3ea3e later
    factored out into invalid_tparm_referent_p), but we need to also remove
    the one in convert_nontype_argument_function for benefit of the first and
    third testcase which we currently reject even in C++17/20 mode.

    And in invalid_tparm_referent_p we're inadvertendly returning false for
    the address of a lambda's static op() since it's DECL_ARTIFICIAL, which
    currently causes us to reject the second (C++20) testcase.  But this
    DECL_ARTIFICIAL check seems to be relevant only for VAR_DECL, and in fact
    this code path was originally reachable only for VAR_DECL until recently
    (r13-6970-gb5e38b1c166357).  So this patch restricts the check to VAR_DECL.

    Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

            PR c++/83258
            PR c++/80488
            PR c++/97700

    gcc/cp/ChangeLog:

            * pt.cc (convert_nontype_argument_function): Remove linkage
            requirement for C++17 and later.
            (invalid_tparm_referent_p) <case ADDR_EXPR>: Restrict
            DECL_ARTIFICIAL rejection test to VAR_DECL.

    gcc/testsuite/ChangeLog:

            * g++.dg/ext/visibility/anon8.C: Don't expect a "no linkage"
            error for the template argument &B2:fn in C++17 mode.
            * g++.dg/cpp0x/lambda/lambda-conv15.C: New test.
            * g++.dg/cpp2a/nontype-class56.C: New test.
            * g++.dg/template/function2.C: New test.

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
   Target Milestone|---                         |13.2

--- Comment #8 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed on trunk so far.

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

--- Comment #9 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:42f9b481be3527b336b800128247d053fd18d121

commit r13-7324-g42f9b481be3527b336b800128247d053fd18d121
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu May 11 10:04:25 2023 -0400

    c++: converted lambda as template argument [PR83258, ...]

    r8-1253-g3d2e25a240c711 removed the template argument linkage requirement
    in convert_nontype_argument for C++17 (which r9-3836-g4be5c72cf3ea3e later
    factored out into invalid_tparm_referent_p), but we need to also remove
    the one in convert_nontype_argument_function for benefit of the first and
    third testcase which we currently reject even in C++17/20 mode.

    And in invalid_tparm_referent_p we're inadvertendly returning false for
    the address of a lambda's static op() since it's DECL_ARTIFICIAL, which
    currently causes us to reject the second (C++20) testcase.  But this
    DECL_ARTIFICIAL check seems to be relevant only for VAR_DECL, and in fact
    this code path was originally reachable only for VAR_DECL until recently
    (r13-6970-gb5e38b1c166357).  So this patch restricts the check to VAR_DECL.

    Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

            PR c++/83258
            PR c++/80488
            PR c++/97700

    gcc/cp/ChangeLog:

            * pt.cc (convert_nontype_argument_function): Remove linkage
            requirement for C++17 and later.
            (invalid_tparm_referent_p) <case ADDR_EXPR>: Restrict
            DECL_ARTIFICIAL rejection test to VAR_DECL.

    gcc/testsuite/ChangeLog:

            * g++.dg/ext/visibility/anon8.C: Don't expect a "no linkage"
            error for the template argument &B2:fn in C++17 mode.
            * g++.dg/cpp0x/lambda/lambda-conv15.C: New test.
            * g++.dg/cpp2a/nontype-class56.C: New test.
            * g++.dg/template/function2.C: New test.

    (cherry picked from commit c3afdb8ba8f1839544c414f57e41a58c8fda5349)

[Bug c++/97700] Bogus error when a class containing a function pointer is used as a non-type template parameter

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97700

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #10 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Should be fully fixed for GCC 13.2