nested functions and closures

[Johannes Pfau]

When debugging gdc I've found some at least for me interesting
behavior which I'm trying to understand now:

----
auto testFunction() {
    int a = 41;
    int nestedFunc() { return ++a; }

    struct NestedStruct {
        int answer() { return nestedFunc(); }
    }

    return NestedStruct();
}

writeln(testFunction().answer());
----

This is valid D code, creates a closure.

----
class testClass
{
    int a = 41;
    int nestedFunc() { return ++a; }

    struct NestedStruct {
        int answer() { return nestedFunc(); }
    }
    
    auto getStruct() { return NestedStruct(); }
}

writeln((new TestClass()).getStruct().answer);
----

This does not compile:
test.d(33): Error: this for nestedFunc needs to be type testClass not
type NestedStruct

Is this intended? It seems to be very similar to the function closure
code. Couldn't NestedStruct just get a pointer to the testClass to
access its variables? Or am I missing something?

Everything works fine if NestedStruct is a class and not a struct so I
guess this functionality was disabled deliberately. But why? And why
wasn't using a nested struct in functions in a similar way disallowed as
well?