Is there anyway to have a if constexpr in expression?

Question

According to https://en.cppreference.com/w/cpp/language/statements, I know I can express condition in C++ by at least the following two ways:

1. An if statement

   if (a) { b } else { c }
   

2. An expression statement

   a ? b : c
   

And we have constexpr for if

if constexpr (a) { b } else { c }

Since in some area, we can only use expression statement for conditions, to avoid getting a "error: expected expression". So I write

cond ? call_func(), 0 : 0

However, how can we involve constexpr this in expression statement? This is a real problem, consider

template <int FLAG>
void f() {
    SOME_MACRO_ONLY_ALLOW_EXPRS({ FLAG ? call_func(), 0 : 0; })
}

I know I can specialize f for FLAG=1 and FLAG=0. However, it may involves lots of modification.