Why libcxx's condition_variable is constexpr and noexcept, but in The Standard it is not?

Question

https://github.com/llvm-mirror/libcxx/blob/master/include/__mutex_base#L290

class _LIBCPP_TYPE_VIS condition_variable
{
    __libcpp_condvar_t __cv_ = _LIBCPP_CONDVAR_INITIALIZER;
public:
    _LIBCPP_INLINE_VISIBILITY
    _LIBCPP_CONSTEXPR condition_variable() _NOEXCEPT = default;

but The Standard declares it as

class condition_variable {
public:
    condition_variable();
    ~condition_variable();

( http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4849.pdf )

And condition_variable() may throw.

Is libcxx incompatible with C++ Standard, or am I wrong?

Answer 1

An implementation is allowed to add noexcept to a non-virtual function if it wont ever throw an exception, see [res.on.exception.handling]/5 of the C++17 standard (draft N4659).

However an implementation is not allowed to add constexpr to a function. See [constexpr.functions]/1. See also LWG issue 2013.

std::condition_variable::condition_variable() is specified as neither constexpr, nor noexcept, but there are no circumstances under which it must throw an exception. See [thread.condition.condvar].

So, noexcept is fine, but constexpr is not. However, functions being marked constexpr that shouldn't be, is a common non-conformance. For example GCC declares math functions deliberately constexpr although they are not supposed to be.