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#include <iostream>
#include <typeinfo>
 
class A {};
class B {};
class C {};
 
template<typename T> 
std::size_t constexpr getID() { return typeid(T).hash_code(); }
 
template<std::size_t N>
struct FF
{
 
};
 
int main()
{
    std::cout << getID<A>() << std::endl;
    std::cout << getID<B>() << std::endl;
    std::cout << getID<C>() << std::endl;
 
	FF<getID<A>()> a;
 
    return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:23:14: error: ‘constexpr std::size_t getID() [with T = A; std::size_t = unsigned int]’ called in a constant expression
  FF<getID<A>()> a;
              ^
prog.cpp:9:23: note: ‘constexpr std::size_t getID() [with T = A; std::size_t = unsigned int]’ is not usable as a constexpr function because:
 std::size_t constexpr getID() { return typeid(T).hash_code(); }
                       ^
prog.cpp:9:60: error: call to non-constexpr function ‘std::size_t std::type_info::hash_code() const’
 std::size_t constexpr getID() { return typeid(T).hash_code(); }
                                                            ^
prog.cpp:23:15: note: in template argument for type ‘unsigned int’ 
  FF<getID<A>()> a;
               ^
prog.cpp:23:18: error: invalid type in declaration before ‘;’ token
  FF<getID<A>()> a;
                  ^
prog.cpp:23:17: warning: unused variable ‘a’ [-Wunused-variable]
  FF<getID<A>()> a;
                 ^

Standard output is empty