importing std.array: empty in a struct messes things up

[Dennis via Digitalmars-d-learn]

I was making a stack interface for an array:
```
struct Stack(T) {
    import std.array: empty;
    T[] stack;
    alias stack this;
}
void main()
{
    Stack!int stack;
    bool x = stack.empty;
}
```
My expectation is that you can now call `empty` on a stack 
instance since I imported it in the struct, but it gives this 
error:
```
Error: cannot resolve type for stack.empty(T)(auto ref scope 
const(T) a) if (is(typeof(a.length) : size_t) || isNarrowString!T)
```

When adding this method to the struct:
```
bool empty() {return stack.empty;}
```

I get this confusing error:
```
Error: expression stack.empty is void and has no value
```

I can solve this by importing std.array: empty in the method 
instead of the struct, but for my understanding of import 
statements in structs I'd appreciate if someone explained what 
these errors mean and why exactly they occur.

Thanks.