Generic lambda cannot be used within a namespace?

Question

Consider the following piece of code

#include <iostream>
#include <functional>

namespace A {
    template<typename T>
    struct X {
        using Function = std::function<int(T)>;
        static Function f;
    };

    template<typename T>
    typename X<T>::Function X<T>::f = [](auto) { return 42; };

}

int main() {
    std::cout << A::X<int>::f(0);
}

Both GCC and clang accept this code but MSVC (tested version 19.00.23506) gives:

error C2888: 'auto <lambda_ce48e25aa4b9e3d225584044e4eae9e2>::operator ()(_T1) const': symbol cannot be defined within namespace 'A'

And indeed, if I remove namespace A and define everything in the global namespace, the code is accepted. Same if I make lambda expression non-generic.

Can someone explain what is the problem that MSVC sees in this code? Does C++ Standard restrict usage of generic lambdas in contexts like above?

Answer 1

Yes, this is a MSVC bug but it has been fixed on VS2017 15.6 preview 2.0

There is no specification in the standard that restricts generic lambdas to exist only in the global namespace.

POC of the problem can be found here: https://godbolt.org/g/BESMK4

MSVC is not able to deduce that auto is exactly T in every case, and fails.

In case you need to make it work with MSVC, replace auto with explicit T:

template<typename T>
typename X<T>::Function X<T>::f = [](T) { return 42; };

https://godbolt.org/g/cYG9GC