I'm a bit surprised by this snippet from the "Traversing and manipulating data" example:

  for (auto& elem : *arr) {
      // visitation helps deal with the polymorphic nature of TOML data
      elem.visit([=](auto&& el) noexcept
      {
          if constexpr (toml::is_number<decltype(el)>)
              (*el)++;
          else if constexpr (toml::is_string<decltype(el)>)
              el = "five"sv;
      });
  }

I thought constexpr would work at compile time but how can the compile here inside a loop "know" when this labmda will encounter an int or a string ?