What is the need of template lambda introduced in C++20 when C++14 already has generic lambda?

Question

c++14 introduced generic lambdas that made it possible to write following:

auto func = [](auto a, auto b){     return a + b; }; auto Foo = func(2, 5); auto Bar = func("hello", "world"); 

It is very clear that this generic lambda func works just like a templated function func would work.

Why did the C++ committee decide to add template syntax for generic lamda?

Answer 1

C++14 generic lambdas are a very cool way to generate a functor with an operator () that looks like this:

template <class T, class U> auto operator()(T t, U u) const; 

But not like this:

template <class T> auto operator()(T t1, T t2) const; // Same type please 

Nor like this:

template <class T, std::size_t N> auto operator()(std::array<T, N> const &) const; // Only `std::array` please 

Nor like this (although this gets a bit tricky to actually use):

template <class T> auto operator()() const; // No deduction 

C++14 lambdas are fine, but C++20 allows us to implement these cases without hassle.

Answer 2

Since you can use templated lambdas in C++20, you can restrict your types in an easier way than an SFINAE expression:

auto lambda = []<typename T>(std::vector<T> t){}; 

This lambda will work only with vector types.