a test failure in ScopeGuardTest.cpp due to missing "const" specifier on VS2022

[ruiyuanlu]

Hi, I found an unexpected failure test case in ScopeGuardTest.cpp on VS2022 today. I'm really confused.

At first, the TESTThrowingCleanupAction test case failed on VS2022 all the time. Then I checked the copy-constructor and found the ScopeGuard use std::ref to wrap a throwable functor.

Interestingly, I noticed that msvc adds "const" specifier to their std::ref implementations as follows:

# in msvc's <type_traits> line 2057-2063
public:
    template <class... _Types>
    _CONSTEXPR20 auto operator()(_Types&&... _Args) const                      // NOTE: const here!!!
        noexcept(noexcept(_STD invoke(*_Ptr, static_cast<_Types&&>(_Args)...)))
            -> decltype(_STD invoke(*_Ptr, static_cast<_Types&&>(_Args)...)) {
        return _STD invoke(*_Ptr, static_cast<_Types&&>(_Args)...);
    }

I guess cosnt/non-const specifier might make things differet. So I added const as:

TEST(ScopeGuard, TESTThrowingCleanupAction) {
  struct ThrowingCleanupAction {
    // clang-format off
    explicit ThrowingCleanupAction(int& scopeExitExecuted)
        : scopeExitExecuted_(scopeExitExecuted) {}
    [[noreturn]] ThrowingCleanupAction(const ThrowingCleanupAction& other)
        : scopeExitExecuted_(other.scopeExitExecuted_) {
      throw std::runtime_error("whoa");
    }
    // clang-format on
    void operator()() const { ++scopeExitExecuted_; }  // NOTE: add const here

   private:
    int& scopeExitExecuted_;
  };
  int scopeExitExecuted = 0;
  ThrowingCleanupAction onExit(scopeExitExecuted);
  EXPECT_THROW((void)makeGuard(onExit), std::runtime_error);
  EXPECT_EQ(scopeExitExecuted, 1);
}

And then everything goes just fine.

But I'm confused:

1. Why non-const version failed?
2. The member var scopeExitExecuted_ could be changed by operator(), why const specifier can be added here? In the past, I thought const before function body means no changes will be made to the members, but it seems incorrect. What does this const actually do?
3. Which version should choose if I need to write sth like this? const or non-const version? Does the chosen version has the portability on other platforms?