The Template Introduction from the Concepts TS is a new way to use concepts. This syntactic variant is not included in the Concepts Draft and, therefore, in the C++20 standard. But, I don't know what the farther away future brings.
Template Introduction
Let me first start with a short riddle. Which one, of the following syntactic variants, is not possible with the draft of the concept?
template<typename T> // (1)
requires Integral<T>
T gcd(T a, T b){
if( b == 0 ) return a;
else return gcd(b, a % b);
}
template<typename T> // (2)
T gcd1(T a, T b) requires Integral<T>{
if( b == 0 ){ return a; }
else return gcd(b, a % b);
}
template<Integral T> // (3)
T gcd2(T a, T b){
if( b == 0 ){ return a; }
else return gcd(b, a % b);
}
Integral auto gcd3(Integral auto a, Integral auto b){ // (4)
if( b == 0 ){ return a; }
else return gcd(b, a % b);
}
Integral{T} // (5)
Integral gcd(T a, T b){
if( b == 0 ){ return a; }
else{
return gcd(b, a % b);
}
}
Maybe, you don't know. So let me name the five variants.
 Requires clause
 Trailing requires clause
 Constrained template parameters
 Abbreviated Function Templates
 Template Introduction
I assume you have chosen the most obscure one: Template Introduction. You are right. Maybe we get it with a later C++ standard, but I'm highly sceptical. In my talks in the last year, I heard no one complaining that they are not part of C++20. I'm also not a big fan of Template Introduction because they introduced a new asymmetry, and you know, I'm not a fan of asymmetries.
The Asymmetry
Instead of declaring your constrained template by using template<Integral T>, you can just Integral{T}. Here, you see the asymmetry. You can only use Template Introduction with concepts (constrained placeholders) but not with auto (unconstrained placeholders). The following example makes my point. This and the following example is based on the previous concepts TS specification and compiled with the GCC.
// templateIntroduction.cpp
#include <type_traits>
#include <iostream>
template<typename T>
concept bool Integral(){
return std::is_integral<T>::value;
}
Integral{T} // (1)
Integral gcd(T a, T b){
if( b == 0 ){ return a; }
else{
return gcd(b, a % b);
}
}
Integral{T} // (2)
class ConstrainedClass{};
/*
auto{T} // (4)
auto gcd(T a, T b){
if( b == 0 ){ return a; }
else{
return gcd(b, a % b);
}
}
auto{T} // (5)
class ConstrainedClass{};
*/
int main(){
std::cout << std::endl;
auto res= gcd(100, 10);
ConstrainedClass<int> constrainedClass;
ConstrainedClass<double> constrainedClass1; // (3)
std::cout << std::endl;
}
I use Template introduction for the function template gcd (line 1) and the class template ConstrainedClass (line 2). As expected, the concept will kick in if I try to instantiate ConstraintedClass for double (line 3).
I don't like it that I can not just replace Integral with auto such as in lines 4 and 5. Up to this point in my posts to concepts, I used constrained placeholders (concepts) and unconstrained placeholders (auto) interchangeably. This straightforward principle is gone with Template Introduction.
Of course, I could easily overcome this restriction by defining a concept that always evaluated to true.
// templateIntroductionGeneric.cpp
#include <iostream>
#include <string>
#include <typeinfo>
#include <utility>
struct NoDefaultConstructor{ // (5)
NoDefaultConstructor() = delete;
};
template<typename T> // (1)
concept bool Generic(){
return true;
}
Generic{T} // (2)
Generic gcd(T a, T b){
if( b == 0 ){ return a; }
else{
return gcd(b, a % b);
}
}
Generic{T} // (3)
class ConstrainedClass{
public:
ConstrainedClass(){
std::cout << typeid(decltype(std::declval<T>())).name() // (4)
<< std::endl;
}
};
int main(){
std::cout << std::endl;
std::cout << "gcd(100, 10): " << gcd(100, 10) << std::endl;
std::cout << std::endl;
ConstrainedClass<int> genericClassInt;
ConstrainedClass<std::string> genericClassString;
ConstrainedClass<double> genericClassDouble;
ConstrainedClass<NoDefaultConstructor> genericNoDefaultConstructor;
std::cout << std::endl;
}
Generic (line 1) is a concept that returns true for all types. Now, I can unify the syntax and define an unconstrained function template (line 4) and an unconstrained class template (line 3). Honestly, the terms unconstrained or constrained function templates or class template are not official. I coined them for simplicity reasons.
The expression typeid(decltype(std::declval<T>())).name() (line 4) may look weird to you. std::declvar<T> (C++11) converts the type parameter T into a reference type. Thanks to the reference type, you can use it in a decltype expression to invoke any member function on T without constructing T. It works even for a type T without a default constructor (line 5). In my case (line 4), I invoked the constructor on the reference type to get the string representation of the type parameter T. Here is the output of the program with GCC.
What's next?
One big topic is left to complete the story to concepts: define your concept. Most of the times, you reinvent the wheel because C++20 has many predefined concepts. Anyway, I present with my next post the predefined concepts and who you can define your own.
This is my last post for 2019. You hear from me on 12.01.2020. If your hopefully contemplative time is too contemplative, here are about 300 posts to modern C++: 

Thanks a lot to my Patreon Supporters: Paul Baxter, Meeting C++, Matt Braun, Roman Postanciuc, Venkata Ramesh Gudpati, Tobias Zindl, Marko, G Prvulovic, Reiner Eiteljörge, Reinhold Dröge, Abernitzke, Richard Ohnemus, Frank Grimm, Sakib, Broeserl, António Pina, Markus Falkner, Darshan Mody, Sergey Agafyin, Андрей Бурмистров, and Jake.
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