Mdtryhht

I implemented a Visit function (on a variant) that checks that the currently active type in the variant matches the function signature (more precisely the first argument). Based on this nice answer.
For example

#include <variant>
#include <string>
#include <iostream>

template<typename Ret, typename Arg, typename... Rest>
Arg first_argument_helper(Ret(*) (Arg, Rest...));

template<typename Ret, typename F, typename Arg, typename... Rest>
Arg first_argument_helper(Ret(F::*) (Arg, Rest...));

template<typename Ret, typename F, typename Arg, typename... Rest>
Arg first_argument_helper(Ret(F::*) (Arg, Rest...) const);

template <typename F>
decltype(first_argument_helper(&F::operator())) first_argument_helper(F);

template <typename T>
using first_argument = decltype(first_argument_helper(std::declval<T>()));

std::variant<int, std::string> data="abc";
template <typename V>
void Visit(V v)
using Arg1 = typename std::remove_const_t<std::remove_reference_t<first_argument<V>>>;//... TMP magic to get 1st argument of visitor + remove cvr, see Q 43526647
if (! std::holds_alternative<Arg1>(data)) 
std::cerr<< "alternative mismatchn";
return;

v(std::get<Arg1>(data));

int main()
 Visit([](const int& i)std::cout << i << "n"; );
 Visit([](const std::string& s)std::cout << s << "n"; );
 // Visit([](auto& x)); ugly kabooom

This works, but it explodes with a user unfriendly compile time error when users passes a generic (e.g. [](auto&)) lambda. Is there a way to detect this and give nice static_assert() about it?
Would also be nice if it worked with function templates as well, not just with lambdas.

Note that I do not know what possible lambdas do, so I can not do some clever stuff with Dummy types since lambdas may invoke arbitrary functions on types.
In other words I can not try to call lambda in 2 std::void_t tests on int and std::string and if it works assume it is generic because they might try to call .BlaLol() on int and string.

Is there a way to detect this and give nice static_assert about it?

I suppose you can use SFINAE over operator() type.

Follows an example

#include <type_traits>

template <typename T>
constexpr auto foo (T const &)
 -> decltype( &T::operator(), bool )
 return true; 

constexpr bool foo (...)
 return false; 

int main()
 
 auto l1 = [](int) return 0; ;
 auto l2 = [](auto) return 0; ;

 static_assert( foo(l1), "!" );
 static_assert( ! foo(l2), "!" );
 

Instead of a bool, you can return std::true_type (from foo() first version) or std::false_type (from second version) if you want to use it through decltype().

Would also be nice if it worked with function templates as well, not just with lambdas.

I don't think it's possible in a so simple way: a lambda (also a generic lambda) is an object; a template function isn't an object but a set of objects. You can pass an object to a function, not a set of objects.

But the preceding solution should works also for classes/structs with operator()s: when there is a single, non template, operator(), you should get 1 from foo(); otherwise (no operator(), more than one operator(), template operator()), foo() should return 0.

Yet another simpler option:

#include <type_traits>
...
template <typename V>
void Visit(V v) 
 class Auto ;
 static_assert(!std::is_invocable<V, Auto&>::value);
 static_assert(!std::is_invocable<V, Auto*>::value);
 ...

The Auto class is just an invented type impossible to occur in the V parameters. If V accepts Auto as an argument it must be a generic.

I tested in coliru and I can confirm the solution covers these cases:

Visit([](auto x)); // nice static assert
Visit([](auto *x)); // nice static assert
Visit([](auto &x)); // nice static assert
Visit([](auto &&x)); // nice static assert

I'm not sure if that would cover all the possible lambdas that you don't know which are :)

#include <variant>
#include <string>
#include <iostream>

template <class U, typename T = void>
struct can_be_checked : public std::false_type ;

template <typename U>
struct can_be_checked<U, std::enable_if_t< std::is_function<U>::value > > : public std::true_type;

template <typename U>
struct can_be_checked<U, std::void_t<decltype(&U::operator())>> : public std::true_type;


template<typename Ret, typename Arg, typename... Rest>
Arg first_argument_helper(Ret(*) (Arg, Rest...));

template<typename Ret, typename F, typename Arg, typename... Rest>
Arg first_argument_helper(Ret(F::*) (Arg, Rest...));

template<typename Ret, typename F, typename Arg, typename... Rest>
Arg first_argument_helper(Ret(F::*) (Arg, Rest...) const);

template <typename F>
decltype(first_argument_helper(&F::operator())) first_argument_helper(F);

template <typename T>
using first_argument = decltype(first_argument_helper(std::declval<T>()));

std::variant<int, std::string> data="abc";


template <typename V>
void Visit(V v)
 if constexpr ( can_be_checked<std::remove_pointer_t<decltype(v)>>::value )
 
 using Arg1 = typename std::remove_const_t<std::remove_reference_t<first_argument<V>>>;//... TMP magic to get 1st argument of visitor + remove cvr, see Q 43526647
 if (! std::holds_alternative<Arg1>(data)) 
 
 std::cerr<< "alternative mismatchn";
 return;
 
 v(std::get<Arg1>(data));
 
 else
 
 std::cout << "it's a template / auto lambda " << std::endl;
 




template <class T>
void foo(const T& t)

 std::cout <<t << " foo n";


void fooi(const int& t)

 std::cout <<t << " fooi " << std::endl;


int main()
 Visit([](const int& i)std::cout << i << std::endl; );
 Visit([](const std::string& s)std::cout << s << std::endl; );
 Visit([](auto& x)std::cout <<x << std::endl;); // it's a template / auto lambda*/
 Visit(foo<int>);

 Visit<decltype(fooi)>(fooi);
 Visit(fooi); 


 // Visit(foo); // => fail ugly

I don't know if it's you want, but you can, with that static_assert if an auto lambda is passed as parameter.

I think it's not possible to do the same for template function, but not sure.