O'Dwyer's Auto macro - hygenic scope guard w/ zero overhead

Auto.h

#pragma once

# O'Dwyer's Auto macro - hygenic scope guard w/ zero overhead
# https://quuxplusone.github.io/blog/2018/08/11/the-auto-macro/
#
# "This scope guard has perfect codegen with zero overhead (at `-O2`) on all major compilers."
# "Because it requires no explicit captures, no novel variable name, no special treatment for `this`,
# it is highly suited for mechanically generated code."
# "It is portable all the way back to C++11."

template<class L>
class AtScopeExit {
  L& m_lambda;
public:
  AtScopeExit(L& action) : m_lambda(action) {}
  ~AtScopeExit() { m_lambda(); }
};

#define TOKEN_PASTEx(x, y) x ## y
#define TOKEN_PASTE(x, y) TOKEN_PASTEx(x, y)

#define Auto_INTERNAL1(lname, aname, ...) \
  auto lname = [&]() { __VA_ARGS__; };    \
  AtScopeExit<decltype(lname)> aname(lname);

#define AUTO_INTERNAL2(ctr,..)                \
  Auto_INTERNAL1(TOKEN_PASTE(Auto_func, ctr), \
                 TOKEN_PASTE(Auto_instance_, ctr), __VA_ARGS__)

#define Auto(...) \
  Auto_INTERNAL2(__COUNTER__, __VA_ARGS__)

# Example:
#   Auto(foo->cleanup());
# Nesting example:
#   Auto(
#     puts("start 1");
#     Auto(puts("cleanup 2"));
#     puts("getting ready to clean up after 1");
#   );

# "I don't always use ad-hoc scope guards, but when I do, I prefer 'auto.h'."