[Bug c++/96106] New: A friend abbreviated template function denies access to private members

[Bug c++/96106] New: A friend abbreviated template function denies access to private members

[kirshamir at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96106

            Bug ID: 96106
           Summary: A friend abbreviated template function denies access
                    to private members
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kirshamir at gmail dot com
  Target Milestone: ---

template<typename T>
class number {
    T num;
public:
    number(T num = 0): num(num) {}

    friend auto add(auto a, auto b);
};

auto add(auto a, auto b) {
    // unjustified compilation error on the line below
    return number{a}.num + number{b}.num;
}

int main() {
    auto result = add(1.0, 2.0);
}

Compilation error provided by gcc (version 10.1 with -std=c++20):

In instantiation of 'auto add(auto:13, auto:14) [with auto:13 = double; auto:14
= double]':
error: 'double number<double>::num' is private within this context
   return number{a}.num + number{b}.num;

See also:
https://stackoverflow.com/questions/62779488/a-friend-abbreviated-template-function-clang-and-gcc-differ

[Bug c++/96106] [10/11 Regression] A friend abbreviated template function denies access to private members

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96106

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|A friend abbreviated        |[10/11 Regression] A friend
                   |template function denies    |abbreviated template
                   |access to private members   |function denies access to
                   |                            |private members
      Known to fail|                            |10.1.0, 11.0
      Known to work|                            |9.3.0
   Last reconfirmed|                            |2020-07-08
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This was accepted with -std=gnu++2a -fconcepts until r276764, "Update the
concepts implementation to conform to C++20."

It's still accepted with -std=gnu++17 -fconcepts now.

[Bug c++/96106] [10/11 Regression] A friend abbreviated template function denies access to private members

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96106

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.2

[Bug c++/96106] [10/11 Regression] A friend abbreviated template function denies access to private members

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96106

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

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

[Bug c++/96106] [10/11 Regression] A friend abbreviated template function denies access to private members

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96106

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.2                        |10.3

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 10.2 is released, adjusting target milestone.

[Bug c++/96106] [10/11 Regression] A friend abbreviated template function denies access to private members

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96106

--- Comment #3 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:f31dd9beb95f4beda1d2bd5c0526c42d0ce455c4

commit r11-2418-gf31dd9beb95f4beda1d2bd5c0526c42d0ce455c4
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jul 29 22:06:36 2020 -0400

    c++: abbreviated function template friend matching [PR96106]

    In the below testcase, duplicate_decls wasn't merging the tsubsted
    friend declaration for 'void add(auto)' with its definition, because
    reduce_template_parm_level (during tsubst_friend_function) lost the
    DECL_VIRTUAL_P flag on the auto's invented template parameter, which
    caused template_heads_equivalent_p to deem the two template heads as not
    equivalent in C++20 mode.

    This patch makes reduce_template_parm_level carry over the
    DECL_VIRTUAL_P flag from the original TEMPLATE_PARM_DECL.

    gcc/cp/ChangeLog:

            PR c++/96106
            * pt.c (reduce_template_parm_level): Propagate DECL_VIRTUAL_P
            from the original TEMPLATE_PARM_DECL to the new lowered one.

    gcc/testsuite/ChangeLog:

            PR c++/96106
            * g++.dg/concepts/abbrev7.C: New test.

[Bug c++/96106] [10/11 Regression] A friend abbreviated template function denies access to private members

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96106

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:745ba1024c653324f9f0b88968c6f8989ed1e093

commit r10-8604-g745ba1024c653324f9f0b88968c6f8989ed1e093
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jul 29 22:06:36 2020 -0400

    c++: abbreviated function template friend matching [PR96106]

    In the below testcase, duplicate_decls wasn't merging the tsubsted
    friend declaration for 'void add(auto)' with its definition, because
    reduce_template_parm_level (during tsubst_friend_function) lost the
    DECL_VIRTUAL_P flag on the auto's invented template parameter, which
    caused template_heads_equivalent_p to deem the two template heads as not
    equivalent in C++20 mode.

    This patch makes reduce_template_parm_level carry over the
    DECL_VIRTUAL_P flag from the original TEMPLATE_PARM_DECL.

    gcc/cp/ChangeLog:

            PR c++/96106
            * pt.c (reduce_template_parm_level): Propagate DECL_VIRTUAL_P
            from the original TEMPLATE_PARM_DECL to the new lowered one.

    gcc/testsuite/ChangeLog:

            PR c++/96106
            * g++.dg/concepts/abbrev7.C: New test.

    (cherry picked from commit f31dd9beb95f4beda1d2bd5c0526c42d0ce455c4)

[Bug c++/96106] [10/11 Regression] A friend abbreviated template function denies access to private members

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96106

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

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

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 10.3+, thanks for the report.