clang not recognizing base class within lambda

I have the following struct MyStruct that calls Foo<int> on construction:

struct MyStruct {
    MyStruct() {
        Foo<int>();
    }

    template<typename T>
    void Foo() {
        []() {
            struct Base {
                Base(int n) {}
            };
            struct Derived : Base {
                // using Base=Base; // needs to be uncommented for Apple LLVM version 9.1.0 (clang-902.0.39.2)
                Derived() :
                    Base(0) // problematic line
                {}
            } derived;
        };
    }
};

I tried to compile this with clang (via godbolt, command line params --std=c++1y):

• 3.6: error: type 'Base' is not a direct or virtual base of 'Derived' (see problematic line in code above)
• 3.7 and newer: compiles (with a warning about unused expressions)

So I expected that with newer versions this should be ok. However, when I tried to compile it on a Mac using XCode9 (which, according to this, uses clang 4.0 (clang --version gives Apple LLVM version 9.1.0 (clang-902.0.39.2))), I get an error from the same line as issued by clang 3.6:

error: member initializer 'Base' does not name a non-static data member or base class

As a workaround, I tried introducing using Base=Base within struct Derived (as indicated), which made it compile via XCode. (As a sidenote: Using the same workaround did not work for clang 3.6 on godbolt.)

So here are my questions:

• Is this a compiler bug? If yes: Is it documented somewhere?
• Is my workaround using Base=Base valid, well-defined C++?