Restriction of explicit object parameter in lambda with capture

In C++23, lambda-expression supports an explicit object parameter (a.k.a. "Deducing this"). I found strange restriction for lambda with capturing at [expr.prim.lambda]/p5.



Given a lambda with a lambda-capture, the type of the explicit object parameter, if any, of the lambda's function call operator (possibly instantiated from a function call operator template) shall be either:



* the closure type,

* a class type derived from the closure type, or

* a reference to a possibly cv-qualified such type.






[Example 2:
struct C {
template
C(T);
};

void func(int i) {
int x = [=](this auto&&) { return i; }(); // OK
int y = [=](this C) { return i; }(); // error
int z = [](this C) { return 42; }(); // OK
}



-- end example]



Question: Why is there such restriction for lambda with capture only? Are there any implementation issues?