[libcxx-commits] [libcxx] [libcxx] makes `pair` conditionally trivially copyable for C++20 (PR #84811)

[Nikolas Klauser via libcxx-commits]

philnik777 wrote:

> > Isn't this an ODR violation when compiling pre-C++20 code and C++20 code in the unstable ABI?
> 
> Technically, yes, but I wonder if it's in the same vein as the ABI break we talked about. Is this a concern for our unstable ABI users?

I think it's fine. I just wanted to call it out so we're all on the same page about this.

> > Why didn't the return type SFINAE not work out?
> 
> SFINAE only works in dependent contexts, and I don't think SMFs can ever be dependent.
> 
> ```c++
> // Not SFINAE
> auto operator=(pair const&)
>   -> enable_if<is_trivially_copy_assignable_v<T> && is_trivially_copy_assignable_v<U>> = default;
> 
> // Not a copy assignment operator; can't be defaulted
> pair& operator=(__conditional_t<is_trivially_copy_assignable_v<X> && is_trivially_copy_assignable_v<Y>,
>                                 pair,
>                                 __nat> const&) = default;
> 
> // Not a copy assignment operator; can't be defaulted
> template<class X = T, class Y = U>
> auto operator=(pair const&)
>   -> enable_if<is_trivially_copy_assignable_v<X> && is_trivially_copy_assignable_v<Y>> = default;
> 
> // defaulted constructors seem to preclude this approach
> pair& operator=(__conditional_t<is_trivially_copy_assignable_v<X> && is_trivially_copy_assignable_v<Y>,
>                                 __nat,
>                                 pair> const&) = delete;
> 
> template<bool = true>
> pair& operator=(__conditional_t<!(is_trivially_copy_assignable_v<X> && is_trivially_copy_assignable_v<Y>),
>                                 pair,
>                                 __nat> const&) { ... }
> ```

Urgh, right. Maybe we could handle it through a base class that is templated on whether we can default the assignment?

https://github.com/llvm/llvm-project/pull/84811