How to access the member of a template parameter's? "Member access to incomplete type"

I'm trying to declare a class "Lambdas" that would provide lambdas (and their type information) to another class "Test". Lambdas also is holding the "this" reference to concrete Test instance for access of Test public members, inside the lambdas. I do it to define the lambdas once and then to deduce types anywhere else through decltype() But I get error: Member access to incomplete type:

template <typename T>
struct LambdasInstances {
    T * self;
    explicit LambdasInstances(T * p) : self(p) {} // CAPTURE Test "this"

    auto genLambda1() {
        return [=](int x){
            self->testVar; // ERROR: Member access to incomplete type
        };
    }
};

class Test3 {
public:
    LambdasInstances<Test3> instances;
    int testVar;

    Test3() : instances(this) {}

    decltype(instances.genLambda1()) varLambda = instances.genLambda1();

    void useLambda() { varLambda(123); }
};