Thanks for the answer, but just so I understand, you said a const-reference binds directly to a r-value as when passing an r-value to a const reference argument. Doesn't this mean that both Clang and GCC compiles this erroneously? To my understanding, the compiler roughly emits the snippet (2) of the following possible snippets? (Where the variables named temp_ correspond to temporaries only existing inside the expression) 1. Defined behavior auto temp_optional = boost::make_optional(std::make_unique<int>(42)); auto temp_uptr = std::move(temp_optional.value()); const auto& x = temp_uptr; ~temp_optional(); // temp_uptr is alive 2. Undefined behavior auto temp_optional = boost::make_optional(std::make_unique<int>(42)); auto temp_uptr = std::move(temp_optional.value()); const auto& x = temp_optional.value(); // Whereas value is empty ~temp_uptr() ~temp_optional(); br /Viktor On Fri, Mar 23, 2018 at 6:21 PM Peter Dimov via Boost <boost@lists.boost.org> wrote:
Viktor Sehr wrote:
Clang (6.0) emits "ud2" instructions for both auto&& and const auto&. If the const auto& can bind directly to an r-value, this must mean Clang is wrong, or am I missing something?
"ud2" is Clang's way of encoding undefined behavior. This is an invalid instruction that crashes. Both auto&& and auto const& do the same thing - bind to the rvalue reference returned by value() &&.
GCC on the other hand emits code for both functions, although with slight difference (I'm not skilled enough in assembler to analyze the GCC output in further detail)
Code example: https://godbolt.org/g/7fjszE <https://godbolt.org/g/uNJoZB
GCC crashes too, in another creative way. It returns *p, where the pointer p is [rsp+8], which is NULL. If you change the functions to
int * func1() { const auto& x = func0().value(); return &*x; }
you'll see that Clang returns NULL and GCC returns the value in [rsp+8] which is also NULL.
So both compilers deliberately dereference a NULL pointer because they see that the reference `x` is no longer valid.
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