[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