[libcxx-commits] [PATCH] D61139: (ABI break) Remove could-be-defaulted SMFs from `stack` and `queue` and `priority_queue`.

[Louis Dionne via Phabricator via libcxx-commits]

ldionne added a comment.

In D61139#1485072 <https://reviews.llvm.org/D61139#1485072>, @Quuxplusone wrote:

> @ldionne: I think adding the macro `_LIBCPP_ABI_CONTAINER_ADAPTERS_ALWAYS_NONTRIVIAL` makes the code "more complicated" because then there will be two different codepaths in need of testing.


If we want to make this change, we __have__ to guard it behind a macro because there's no way we're breaking libc++'s ABI v1 for that. And we should already have testers for the libc++ ABI v2 (if someone cares about it), so that shouldn't be a significant increase in complexity.

> [...]
> 
> However! I think the problem with that idea is that it would make `std::stack<int, fixed_capacity_vector<int, 10>>` no longer trivially-destructible! Right now it's not trivially copyable but it is trivially destructible; after your idea, it would remain non-trivially copyable but it would no longer be trivially destructible. So your idea would sadly be a perf //regression//, not merely the desired perf non-improvement. :(

Are you sure about that? cppreference says <https://en.cppreference.com/w/cpp/language/destructor#Trivial_destructor>:

> The destructor for class T is trivial if all of the following is true:
> 
> - The destructor is not user-provided (meaning, it is either implicitly declared, or explicitly defined as defaulted on its first declaration)
> - [...]

In that case, the destructor would be explicitly defined as defaulted on its first declaration (and all the other bullets are satisfied too).


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