The rule is from P0292R2.

Here's an example, adjusted from the discussions on P0292R2 in Oulu. Would you like to see this in the standard?

template<class T>
int f(T x) {
  int v = 10;
  auto lam = [&] (auto a) {
    if constexpr (sizeof(T) > 1)
       ++v;
  };
  lam(5);
  return 0;
}
int v = f('a');   // "v" is not captured

Here's an example, adjusted from the discussions on P0292R2 in Oulu. Would you like to see this in the standard?

template<class T>
int f(T x) {
  int v = 10;
  auto lam = [&] (auto a) {
    if constexpr (sizeof(T) > 1)
       ++v;
  };
  lam(5);
  return 0;
}
int v = f('a');   // "v" is not captured

I guess it might be better to have two examples, one doesn't captures the outer variable like this, and the other captures it because the condition in if constexpr depends on the type of the generic lambda parameter.