[libcxx-commits] [libcxx] [libc++] Overhaul the PSTL dispatching mechanism (PR #88131)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___PSTL_DISPATCH_H
+#define _LIBCPP___PSTL_DISPATCH_H
+
+#include <__config>
+#include <__pstl/backend_fwd.h>
+#include <__type_traits/conditional.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/type_identity.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+namespace __pstl {
+
+template <template <class, class> class _Algorithm, class _Backend, class _ExecutionPolicy, class = void>
+struct __is_implemented : false_type {};
+
+template <template <class, class> class _Algorithm, class _Backend, class _ExecutionPolicy>
+struct __is_implemented<_Algorithm,
+                        _Backend,
+                        _ExecutionPolicy,
+                        __enable_if_t<sizeof(_Algorithm<_Backend, _ExecutionPolicy>)>> : true_type {};
+
+// Helpful to provide better error messages. This will show the algorithm and the execution policy
+// in the compiler diagnostic.
+template <template <class, class> class _Algorithm, class _ExecutionPolicy>
+constexpr bool __cant_find_backend_for = false;
+
+template <template <class, class> class _Algorithm, class _BackendConfiguration, class _ExecutionPolicy>
+struct __find_first_implemented;
+
+template <template <class, class> class _Algorithm, class _ExecutionPolicy>
+struct __find_first_implemented<_Algorithm, __backend_configuration<>, _ExecutionPolicy> {
+  static_assert(__cant_find_backend_for<_Algorithm, _ExecutionPolicy>,
+                "Could not find a PSTL backend for the given algorithm and execution policy");
----------------
ldionne wrote:

As the comment above says:

```c++
// Helpful to provide better error messages. This will show the algorithm and the execution policy
// in the compiler diagnostic.
```

This makes a good difference when hitting a compiler error due to a misconfigured backend.

https://github.com/llvm/llvm-project/pull/88131