decltype fails on struct member declared in template class

Question

I am having an issue with code failing to compile for an external library I am using. I believe that library compiles fine with gcc, but it fails to compile for me with clang.

I can recreate the issue as follows

template <class T>
class A {
public:
    struct B {
        int a;
    };

    void test();

private:
    T _t;
};

template <class T>
void A<T>::test()
{
    printf("Result %d", std::numeric_limits<decltype(B::a)>::max());
}

int main(int argc, char** argv)
{
    auto t = A<int>();
    t.test();
    return 0;
}

This fails to compile on clang with the following error

error: invalid use of non-static data member 'a' printf("Result %d", std::numeric_limits<decltype(B::a)>::max());

My questions are as follows:

• What is the expected behavior?

• decltype on non-static members was added in c++11. Does this apply to those declared in template classes?

• Is this a compiler bug? Or an example of non-conformant code working with gcc?

Answer 1

It was a bug in old version of Clang, and fixed in Clang 3.9.0: https://godbolt.org/g/zqFxL2

Normative Standarteese:

8.2.3: In some contexts, unevaluated operands appear ([expr.prim.req], [expr.typeid], [expr.sizeof], [expr.unary.noexcept], [dcl.type.simple], [temp]). An unevaluated operand is not evaluated. [ Note: In an unevaluated operand, a non-static class member may be named ([expr.prim]) and naming of objects or functions does not, by itself, require that a definition be provided ([basic.def.odr]). An unevaluated operand is considered a full-expression. — end note ]