[shultays]

   The optimizer will delete bar() if FOO is a manifest constant that is not 3. 

Yes, but the compiler won't delete it and complain if bar is not defined. if constexpr is not a direct replacement for if macro

[WalterBright]

> the compiler won't delete it and complain if bar is not defined

    extern void bar();

will define it to the satisfaction of the compiler. The linker won't complain if the compiler removes the call to it.

[shultays]

And then you have the issue of not having types. consider this

  void my_bar_class_type::bar(my_bar_type<my_bar_type_2>::my_bar_inside_type paramater);

and you are calling something like

   myObject->bar(mySecondObject.getWhatever());

You have to mock up like everything

[WalterBright]

Or go up an abstraction layer so the type details are not needed.