[Bug c++/56643] New: Failure to match noexcept specifier of friend template function in template class

[Bug c++/56643] New: Failure to match noexcept specifier of friend template function in template class

[st at quanttec dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56643

             Bug #: 56643
           Summary: Failure to match noexcept specifier of friend template
                    function in template class
    Classification: Unclassified
           Product: gcc
           Version: 4.7.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: st@quanttec.com


The current SVN version of g++ fails to match the noexcept specifier of the
friend declaration and definition of the test template function in the
following code:

template <int N>
struct Test {
    template <int M>
    friend void test(Test<M>& arg) noexcept(M == 0);
};

template <int N>
void test(Test<N>& arg) noexcept(N == 0) {}

int main() {
    Test<0> t;
    test(t);
    return 0;
}

I get the following error:

g++ --std=c++11 test.cpp
test.cpp: In instantiation of ‘struct Test<0>’:
test.cpp:12:13:   required from here
test.cpp:5:17: error: declaration of ‘template<int M> void test(Test<N>&)
noexcept (<uninstantiated>)’ has a different exception specifier
     friend void test(Test<M>& arg) noexcept(M == 0);
                 ^
test.cpp:9:6: error: from previous declaration ‘template<int N> void
test(Test<N>&) noexcept ((N == 0))’
 void test(Test<N>& arg) noexcept(N == 0) {}
      ^

The error message seems to suggest that g++ confuses the template parameter of
the friend function declaration and the template parameter of the containing
class (note the 'N' in the line starting with 'test.cpp:5:17: error').

g++ 4.7.2 reports a similar error.

[Bug c++/56643] Failure to match noexcept specifier of friend template function in template class

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56643

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-03-17
            Version|4.7.3                       |4.9.0
     Ever Confirmed|0                           |1

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2013-03-17 20:27:02 UTC ---
Confirmed.

[Bug c++/56643] Failure to match noexcept specifier of friend template function in template class

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56643

--- Comment #2 from Casey Carter <Casey at Carter dot net> ---
For future readers, I believe the simplest workaround is to make the friend
function a static member function of a new class, friend that class instead of
the function, and introduce a simple forwarding function in the original
function's place:

template <int N>
struct Test {
    friend struct workaround_56643;
};

struct workaround_56643 {
    template <int N>
    static void test(Test<N>&) noexcept(N == 0) {}
};

template <int N>
auto test(Test<N>& arg)
  noexcept(noexcept(workaround_56643::test(arg)))
        -> decltype(workaround_56643::test(arg)) {
             return workaround_56643::test(arg);
}

int main() {
    Test<0> t;
    test(t);
}