struct TTestData {
    TA A;
    TB B;
    ... // next fields
};

struct TTestState : private TTestData {
    ... // some mutable methods
    const TTestData& GetData() const {
        return *static_cast<const TTestData*>(this);
    }
};

void SomeTest {
    TTestState test;
    const auto& [a, b, ...] = test.GetData();
    test.MutableMethod1(...)
        .MutableMethod2(...)
        ...
        .MutableMethodN(...);
}