Why can't templates be declared in a function?

Question

Reading C++ Templates: The Complete Guide and it says

Note that templates cannot be declared in a function

It does not give e

Answer 1

The answer "because standard says so", is of course correct, but let's consider generic lambdas.

In C++14 and C++17 generic lambdas are the only way of writing template-like code that I know of:

    auto lambda = [](auto x) { };
    lambda.operator()(0);

Technically, you can write any kind of template code just with that. Though you'll have to work hard to work around various limitations of this approach.

That will be simpler in C++20 though. With template parameter list in generic lambdas you will be able to write code like this:

    auto size = []() { return sizeof(T); };
    static_assert(4 == size.operator()());

GCC already supports this syntax.