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I recently ran into an interesting Visual Studio bug. Namely, if you have nested namespaces and are making us of a using statement, it seems to mess up namespace lookup.

namespace A {
    struct X  {};

    namespace B {
        struct Y {
            void fun (X &);
        };
    }
}

using namespace A::B;

void Y::fun(X &) {}

int main () {
    A::B::Y y;
}

This code compiles fine with clang and gcc, but for whatever reason, doesn’t compile under Visual Studio. You’ll see errors similiar to:

missing type specifier - int assumed. Note: C++ does not support default-int
syntax error: missing ',' before '&'
'void A::B::Y::fun(X &)': overloaded member function not found in 'A::B::Y'

It seems to overwrite the root namespace scoping (in this case, A, making A::X not found). This is fixable by explicitly writing all namespaces like such:

namespace A {
    struct X  {};

    namespace B {
        struct Y {
            void fun (X);
        };
    }
}

void A::B::Y::fun(X) {}

int main () {
    A::B::Y y;
}

However, this can be a huge pain if you have a lot of references and can really clutter your code. Fortunately, there’s another easy fix, redeclare the root namespace:

using namespace A;

And that should fix it. Thanks to those over at StackOverflow who helped me pinpoint this issue. I’ve also submitting a bug for the Visual Studio folks.