vs10 will compile the following without errors, even when language extensions are disabled.

    namespace A {
        template <typename T>
        class X;
    }

    namespace B {
        template <>
        class A::X<int>;
    }

clang correctly outputs

    error: class template specialization of 'X' must originally be declared in namespace 'A'

The diagnostic should be suppressed when -fms-compatibility is in effect.