Implementing C++20 support

[Daniel Marjamäki]

I am looking at our C++20 support. I have some trouble that I wonder if somebody can help out with.

I fail to create a compilable example for this feature: https://github.com/AnthonyCalandra/modern-cpp-features/blob/master/CPP20.md#template-syntax-for-lambdas. I assume something like this should work:

int foo() {
    auto zero = []<class T>() { return T(0); };
    return zero<int>();
}

[Daniel Marjamäki]

I would also like help with this: https://github.com/AnthonyCalandra/modern-cpp-features/blob/master/CPP20.md#class-types-in-non-type-template-parameters

I fail to compile the code below:

struct foo {
  foo() = default;
  constexpr foo(int) {}
};

template <foo f>
auto get_foo() {
  return f;
}

int main() {
    get_foo(); // uses implicit constructor
    get_foo<foo{123}>();
    return 0;
}