To avoid redundant errors, you must dispatch to an empty implementation on concept check failure:
template <typename I, typename T> I find_impl(std::true_type, I first, I last, const T& value) { while (first != last && *first != value) ++first; }
template <typename I, typename T> I find_impl(std::false_type, I first, I last, const T& value) { static_assert (Input_iterator<I>(), ""); }
template <typename I, typename T> I find(I first, I last, const T& value) { find_impl(Input_iterator<I>(), first, last, value); }
Not if you use the trick above.
Interesting... That looks like it will work. Do you still need the sfinae_error class to get shallow errors? I'm guessing not in these cases.
You only have the guard the calls that add constraints. Other calls will simply propagate sfinae_error and don't need to be guarded. (In my example, S2 doesn't need to guard the call to S1, but S1 must guard S0 because it adds an Addable constraint.) Also, you can guard your function object once by defining it with try_call_wrapper, so it doesn't need to be guarded everywhere.
Just apply more force. :-)
Indeed :) Side note: sfinae_error is a bad name in a worse way than how "PIN number" is redundant. It's not an error, but it is. substitution_failure or subst_failure might be better choices.