[Bug c++/53856] New: Default argument allowed on member defined outside of class template

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53856

             Bug #: 53856
           Summary: Default argument allowed on member defined outside of
                    class template
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: redi@gcc.gnu.org


template<typename T>
struct A
{
    struct B;
};

template<typename T = int>
struct A<T>::B
{
    int i;
};

int main()
{
    A<int>::B b = { };
    return b.i;
}

This should be rejected according to [temp.param]/9

"A default template-argument shall not be specified in the
template-parameter-lists of the definition of a member of a class template that
appears outside of the member’s class."

All versions of G++ since at least 3.4 accept the code above.

Comeau online rejects it:

"ComeauTest.c", line 7: error: a default template argument cannot be specified
on
          the declaration of a member of a class template outside of its class
  template<typename T = int>
                    ^

As does Clang++

bug2.cc:7:19: error: cannot add a default template argument to the definition
of a member of a class template
template<typename T = int>
                  ^   ~~~

Solaris CC accepts it.