implement allocator concept (#1750)

---------

Co-authored-by: Eric Niebler <[email protected]>

Author: Cra3z

include/stdexec/__detail/__concepts.hpp

@@ -271,4 +271,23 @@ namespace STDEXEC {
   template <class _Ty, class... _Us>
     requires __none_of<_Ty, _Us...>
   using __unless_one_of_t = _Ty;
+
+  namespace __detail {
+    template <class _Alloc>
+    auto __test_alloc_pointer(int) -> typename _Alloc::pointer;
+    template <class _Alloc>
+    auto __test_alloc_pointer(long) -> typename _Alloc::value_type*;
+
+    template <class _Alloc>
+    using __alloc_pointer_t = decltype(__detail::__test_alloc_pointer<__decay_t<_Alloc>>(0));
+  } // namespace __detail
+
+  template <class _Alloc>
+  concept __allocator_ = //
+    requires(__decay_t<_Alloc>& __alloc, std::size_t __bytes) {
+      { __alloc.allocate(__bytes) } -> same_as<__detail::__alloc_pointer_t<_Alloc>>;
+      __alloc.deallocate(__alloc.allocate(__bytes), __bytes);
+    } //
+    && copy_constructible<__decay_t<_Alloc>> //
+    && equality_comparable<__decay_t<_Alloc>>;
 } // namespace STDEXEC

include/stdexec/__detail/__queries.hpp

@@ -27,10 +27,6 @@ namespace STDEXEC {
   //////////////////////////////////////////////////////////////////////////////////////////////////
   // [exec.queries]
   namespace __queries {
-    // TODO: implement allocator concept
-    template <class _T0>
-    concept __allocator_c = true;
-
     //////////////////////////////////////////////////////////////////////////////////
     // [exec.get.allocator]
     struct get_allocator_t : __query<get_allocator_t> {
@@ -44,7 +40,7 @@ namespace STDEXEC {
       STDEXEC_ATTRIBUTE(always_inline, host, device)
       static constexpr void __validate() noexcept {
         static_assert(__nothrow_callable<get_allocator_t, const _Env&>);
-        static_assert(__allocator_c<__call_result_t<get_allocator_t, const _Env&>>);
+        static_assert(__allocator_<__call_result_t<get_allocator_t, const _Env&>>);
       }
 
       STDEXEC_ATTRIBUTE(nodiscard, always_inline, host, device)