Static templated constexpr nested class member

Question

I have the following sample class Foo with nested class Bar and everything is constexpr:

class Foo
{
private:
    template <typename T>
    struct Bar
    {
        constexpr Bar(){}
        constexpr int DoTheThing() const
        {
            return 1;
        }
    };
        
public:
    constexpr static auto b = Bar<int>{};
    constexpr Foo() {}
    constexpr int DoTheThing() const
    {
        return b.DoTheThing();
    }
};

And I want to test that calling Foo::DoTheThing returns 1:

int main()
{
   constexpr Foo f;
   static_assert(f.DoTheThing() == 1, "DoTheThing() should return 1");
}

GCC and Clang both complain here, but MSVC does not

GCC says:

error: constexpr Foo::Bar<T>::Bar() [with T = int] used before its definition

constexpr static auto b = Bar<int>{};

And Clang:

error: constexpr variable b must be initialized by a constant expression

constexpr static auto b = Bar<int>{};

I cannot tell if the standard disallows this, but my guess is that somehow b is an incomplete type.

What makes things more interesting is that I can get GCC and Clang to behave if I remove the constexpr, or if I move the definition of Bar outside of Foo.

Which of these compilers is correct?

Note that this question was inspired by the following:

• Simple constexpr LookUpTable in C++14 (my problem is one part of this unanswered question's problem)
• Nested struct breaks constexpr despite being identical to global ones (this seems to provide some insight into what's going on)

Answer 1

From n4140

§ 9.2.2 [class.mem] (Emphasis mine)

A class is considered a completely-defined object type (3.9) (or complete type) at the closing } of the class-specifier. Within the class member-specification, the class is regarded as complete within function bodies, default arguments, using-declarations introducing inheriting constructors (12.9), exception-specifications, and brace-or-equal-initializers for non-static data members (including such things in nested classes). Otherwise it is regarded as incomplete within its own class member-specification.

Clang and GCC are correct. The class is not considered complete when you are declaring your static constexpr member, so you cannot construct it. This is why moving the definition of Bar out or removing the static constexpr works (because it is considered complete when defining non-static members)

---

To clarify, especially considering this question: Static constexpr member of an inner class

The standardese I quoted above basically means that unless otherwise specified a class is regarded incomplete within itself ^*. A static, constexpr, or static constexpr initializer does not fall under the otherwise specified portion, and therefore we can not use anything declared within the class, which includes a nested class type.

^{*meaning you can't use it or members of it within the class declaration. The most well known exception to that is within a member function.}