The following code does not compile. Note that the exact same functions are present in both the inner and outer structs, and that opIndex works for both.

struct Foo{
    struct Bar {
        int opIndex( size_t index ) {
            return 3;
        }
        
        void opIndexAssign( int value, size_t index ) {
        }
    }
    @property
    auto bar( ) {
        return Bar( );
    }
    
    int opIndex( size_t index ) {
        return 3;
    }
    
    void opIndexAssign( int value, size_t index ) {
    }
}

void main( ) {
    Foo f;
    f[3] = f[2];
    Foo.Bar b;
    b[3] = b[2];
    f.bar[3] = f.bar[2]; // Fails
}


foo.d(28): Error: f.bar()[3u] is not an lvalue