I never said I make it SFINAE-friendly! I just said I use enable_if. Sometimes I use enable_if because it's handier for me than an alternative like static_assert, that's all. I actually don't recall if I've ever needed this in an SFINAE context before.

That said, here's one possible place where it might make sense with SFINAE. Note that I haven't fully thought this through, I'm just doing this off the top of my head right now, so it's possible I'll miss something:

  #include <iterator>
  #include <type_traits>

  namespace stdext { template<class> class checked_array_iterator; }  // Forward-declare MSVC-specific type

  namespace bar {
  namespace {

  template<class T>
  std::enable_if_t<(std::is_same_v<stdext::checked_array_iterator<typename std::iterator_traits<typename T::foo_iterator>::value_type *>, typename T::foo_iterator> && ((void)sizeof(typename T::foo_iterator), true))> debug_verify(const T &obj, ...) {
    // extra sanity checks against obj.get_foo_iterator()
  }

  template<class T>
  std::enable_if_t<!(std::is_same_v<stdext::checked_array_iterator<typename std::iterator_traits<typename T::foo_iterator>::value_type *>, typename T::foo_iterator> && ((void)sizeof(typename T::foo_iterator), true))> debug_verify(const T &) {
    // nothing to do
  }
  }

  class S {
    auto foo() {
      bar::debug_verify(this->field1);
      bar::debug_verify(this->field2);
    }
  
    // ...
  };
  }

Note that I'm by no means claiming this is the only way to do this.

(Edit: Removed a final comment about turning it into a is_checked_array_iterator trait - messed up that part.)