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From: "kirshamir at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96106] New: A friend abbreviated template function denies
 access to private members
Date: Wed, 08 Jul 2020 06:57:08 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 10.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96106

            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 =3D 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 =3D add(1.0, 2.0);
}

Compilation error provided by gcc (version 10.1 with -std=3Dc++20):

In instantiation of 'auto add(auto:13, auto:14) [with auto:13 =3D double; a=
uto:14
=3D 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=