[libcxx-commits] [libcxx] [libcxx] changes `__is_allocator` from a struct to a variable or concept (PR #68629)

[Christopher Di Bella via libcxx-commits]

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@@ -21,13 +21,21 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if _LIBCPP_STD_VER >= 20
+template <class _Alloc>
+concept __is_allocator = requires(_Alloc __a) {
+  typename _Alloc::value_type;
+  __a.allocate(size_t(0));
+};
+#else
 template <typename _Alloc, typename = void, typename = void>
-struct __is_allocator : false_type {};
+inline static const bool __is_allocator = false;
 
 template <typename _Alloc>
-struct __is_allocator<_Alloc,
-                      __void_t<typename _Alloc::value_type>,
-                      __void_t<decltype(std::declval<_Alloc&>().allocate(size_t(0)))> > : true_type {};
+inline static const bool __is_allocator<_Alloc,
+                                        __void_t<typename _Alloc::value_type>,
+                                        __void_t<decltype(std::declval<_Alloc&>().allocate(size_t(0)))> > = true;
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cjdb wrote:

I kind of prefer the original way, since it keeps them separate. What does combining afford us?

https://github.com/llvm/llvm-project/pull/68629