[PATCH] D68364: Implement C++20's P0784 (More constexpr containers)

[Louis Dionne via Phabricator via cfe-commits]

ldionne added a comment.

In D68364#2294569 <https://reviews.llvm.org/D68364#2294569>, @jwakely wrote:

> In D68364#2293971 <https://reviews.llvm.org/D68364#2293971>, @leonardchan wrote:
>
>> Is there a recommended way for working around this? We're using GCC 10.2.1. Thanks.
>
> I don't think your implementation is valid. I think P0784 only allows new-expressions and calls to `std::allocator::allocate` in constexpr functions, not calls to `operator new`.

Hmm, right. It's actually impossible to implement `__libcpp_allocate` in a constexpr-friendly manner, since it's just allocating bytes.

> Can you use something like `if (__builtin_is_constant_evaluated()) return new unsigned char[n];` and the equivalent in the corresponding deallocation function?

That doesn't work, since we can't use `unsigned char`s as storage for arbitrary `T`s in `constexpr`.

I fixed it by marking `__libcpp_allocate` as non-`constexpr` and calling `operator new` in `allocator<T>::allocate`, which seems to work on GCC.


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