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There's nothing as elegant as `if constexpr`, unfortunately. However, it is possible to achieve the practical effects (member functions and data only present conditionally).

Start by creating a class template that will encapsulate all the `fooBar`-specific code and data. Use the [Curiously Recurring Template Pattern (CRTP)](https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern) to make the rest of the `Widget` class accessible:
```
template <class Self>
struct Widget_FooBar
{
  void activateFooBar()
  {
    fooBar.activate();
    self.doSomething();
  }

private:
  Self& self() { return static_cast<Self&>(*this); }
  const Self& self() const { return static_cast<const Self&>(*this); }

  FooBar fooBar;
};
```

Next, create an empty class to use as an alternative when `FooBar` is not supposed to be used:
```
struct Widget_NoFooBar
{};
```

Finally, choose the appropriate base class for `Widget`:
```
constexpr bool HAS_FOOBAR = whatever;

struct Widget : std::conditional_t<HAS_FOOBAR, Widget_FooBar<Widget>, Widget_NoFooBar>
{
  void doSomethingElse()
  {
    do_stuff();
    if constexpr(HAS_FOOBAR) {
      activateFooBar();
    }
  }
};
```

This way, the member functions and data are held in `Widget_FooBar`, which is only included in `Widget` if `HAS_FOOBAR` is true.