[libcxx-commits] [libcxx] [libc++] Support sorting consteval-only ranges (PR #134623)

[A. Jiang via libcxx-commits]

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@@ -912,30 +900,63 @@ using __sort_is_specialized_in_library _LIBCPP_NODEBUG = __is_any_of<
     long double>;
 
 template <class _AlgPolicy, class _Type, __enable_if_t<__sort_is_specialized_in_library<_Type>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI void __sort_dispatch(_Type* __first, _Type* __last, __less<>&) {
-  __less<_Type> __comp;
-  std::__sort<__less<_Type>&, _Type*>(__first, __last, __comp);
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __sort_dispatch(_Type* __first, _Type* __last, __less<>&) {
+#if _LIBCPP_STD_VER >= 20
+  if (std::is_constant_evaluated()) {
+    auto __depth_limit = static_cast<ptrdiff_t>(2 * std::__bit_log2(static_cast<size_t>(__last - __first)));
+    std::__introsort<_ClassicAlgPolicy, ranges::less, _Type*, __use_branchless_sort<ranges::less, _Type*>>(
+        __first, __last, ranges::less{}, __depth_limit);
+  } else
+#endif
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frederick-vs-ja wrote:

I guarded this with `_LIBCPP_STD_VER >= 20` because `ranges::less` is used in `__sort` and I wanted the same operations to be performed. And given `_LIBCPP_STD_VER >= 20` is required, IMO its a bit better to use `std::is_constant_evaluated`. (Or even C++23 `if consteval`?)

> IIUC you claim this has got something to do with instantiating an undefined template, but this is instantiating `__sort` exactly the same as the old code.

No, the changes in `__sort_dispatch` didn't change instantiation of `__sort`. I just attempted to add constexpr-friend branches to them.

https://github.com/llvm/llvm-project/pull/134623