Access checks and copy elision before c++17

[cor3ntin]

We claim to support guaranteed copy elision in C++03, but we still perform access checks for copy constructors (before c++17)
https://godbolt.org/z/WPEh8e94c

class S {
public:
S(int, int);

// private:
S(const S &);
};

S s = S(0, 0);

If we make the constructor public, RVO is performed https://godbolt.org/z/WPEh8e94c

Aditionally __cpp_guaranteed_copy_elision is not set even if the documentation claim it should be
https://clang.llvm.org/docs/LanguageExtensions.html#id34

Maybe the documentation is just wrong.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Corentin Jabot (cor3ntin)

We claim to support guaranteed copy elision in C++03, but we still perform access checks for copy constructors (before c++17) https://godbolt.org/z/WPEh8e94c

class S {
public:
S(int, int);

// private:
S(const S &);
};

S s = S(0, 0);

If we make the constructor public, RVO is performed https://godbolt.org/z/WPEh8e94c

Aditionally __cpp_guaranteed_copy_elision is not set even if the documentation claim it should be
https://clang.llvm.org/docs/LanguageExtensions.html#id34

Maybe the documentation is just wrong.

[cor3ntin]

Part of TryConstructorInitialization is gated on C++17 - removing that does break ~70 tests.
However as far as i can tell it is the only place where NRVO is gated on language mode.

[pinskia]

I thought the copy constructor access was still required before C++17 even if NRVO was done.