[libcxx-commits] [libcxx] [libc++][type_traits] P2647R1: A trait for implicit lifetime types (PR #106870)

[Mital Ashok via libcxx-commits]

MitalAshok wrote:

The implementation without the builtin can never be implemented correctly for all structs. You wouldn't be able to check if this class has a trivial default constructor:

```c++
struct X {
  X() = default;  // Trivial eligible constructor
  X(int = 0);  // ... But X is not default constructible
  X(const X&);
};

static_assert(__builtin_is_implicit_lifetime(X));
static_assert(!is_implicit_lifetime_v<X>);
```

Not sure you should try to provide a fallback in the `!__has_builtin` case.

(But if you do keep it, you don't check if it's a class before you check `trivially_destructible&&(trivially_X_constructible||...)`. This allows extension types like `double _Complex` and `int [[vector_size(4)]]`, but does not deny references like `int&` which are *not* implicit lifetime)

https://github.com/llvm/llvm-project/pull/106870