From b54d94cbd16a3532a920daf5d546bf19a2dc1e4c Mon Sep 17 00:00:00 2001
From: Eric Niebler <eniebler@nvidia.com>
Date: Sat, 23 Nov 2024 19:36:46 +0000
Subject: [PATCH] add a `_CCCL_NO_CONCEPTS` config macro

---
 libcudacxx/include/cuda/std/__cccl/dialect.h           |  5 +++++
 .../include/cuda/std/__concepts/concept_macros.h       | 10 +++++-----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/libcudacxx/include/cuda/std/__cccl/dialect.h b/libcudacxx/include/cuda/std/__cccl/dialect.h
index 8dfedd5a3cc..9995a7aea5d 100644
--- a/libcudacxx/include/cuda/std/__cccl/dialect.h
+++ b/libcudacxx/include/cuda/std/__cccl/dialect.h
@@ -104,6 +104,11 @@
 #  define _CCCL_NO_VARIABLE_TEMPLATES
 #endif // _CCCL_STD_VER <= 2011
 
+// concepts are only available from C++20 onwards
+#if _CCCL_STD_VER <= 2017 || !defined(__cpp_concepts) || (__cpp_concepts < 201907L)
+#  define _CCCL_NO_CONCEPTS
+#endif // _CCCL_STD_VER <= 2017
+
 // noexcept function types are only available from C++17 onwards
 #if _CCCL_STD_VER >= 2017 && defined(__cpp_noexcept_function_type) && (__cpp_noexcept_function_type >= 201510L)
 #  define _CCCL_FUNCTION_TYPE_NOEXCEPT noexcept
diff --git a/libcudacxx/include/cuda/std/__concepts/concept_macros.h b/libcudacxx/include/cuda/std/__concepts/concept_macros.h
index 2850c38a493..1301f3f6d59 100644
--- a/libcudacxx/include/cuda/std/__concepts/concept_macros.h
+++ b/libcudacxx/include/cuda/std/__concepts/concept_macros.h
@@ -50,19 +50,19 @@ using __cccl_enable_if_t = typename __cccl_select<_Bp>::template type<_Tp>;
 template <class _Tp, bool _Bp>
 using __cccl_requires_t = typename __cccl_select<_Bp>::template type<_Tp>;
 
-#if (defined(__cpp_concepts) && _CCCL_STD_VER >= 2020)
+#if !defined(_CCCL_NO_CONCEPTS) || defined(_CCCL_DOXYGEN_INVOKED)
 #  define _CCCL_TEMPLATE(...)               template <__VA_ARGS__>
 #  define _CCCL_REQUIRES(...)               requires __VA_ARGS__
 #  define _CCCL_AND                         &&
 #  define _CCCL_TRAILING_REQUIRES_AUX_(...) requires __VA_ARGS__
 #  define _CCCL_TRAILING_REQUIRES(...)      ->__VA_ARGS__ _CCCL_TRAILING_REQUIRES_AUX_
-#else // ^^^ __cpp_concepts ^^^ / vvv !__cpp_concepts vvv
+#else // ^^^ _CCCL_NO_CONCEPTS ^^^ / vvv !_CCCL_NO_CONCEPTS vvv
 #  define _CCCL_TEMPLATE(...)               template <__VA_ARGS__
 #  define _CCCL_REQUIRES(...)               , bool _CCCL_true_ = true, __cccl_enable_if_t < __VA_ARGS__ && _CCCL_true_, int > = 0 >
 #  define _CCCL_AND                         &&_CCCL_true_, int > = 0, __cccl_enable_if_t <
 #  define _CCCL_TRAILING_REQUIRES_AUX_(...) , __VA_ARGS__ >
 #  define _CCCL_TRAILING_REQUIRES(...)      ->__cccl_requires_t < __VA_ARGS__ _CCCL_TRAILING_REQUIRES_AUX_
-#endif // !__cpp_concepts
+#endif // !defined(_CCCL_NO_CONCEPTS)
 
 #if _CCCL_STD_VER >= 2014
 
@@ -145,7 +145,7 @@ namespace __unqualified_cuda_std = _CUDA_VSTD; // NOLINT(misc-unused-alias-decls
 #  define _CCCL_PP_EAT_TYPENAME_SELECT_1(...) _CCCL_PP_CAT3(_CCCL_PP_EAT_TYPENAME_, __VA_ARGS__)
 #  define _CCCL_PP_EAT_TYPENAME_typename
 
-#  if (defined(__cpp_concepts) && _CCCL_STD_VER >= 2020) || defined(_CCCL_DOXYGEN_INVOKED)
+#  if !defined(_CCCL_NO_CONCEPTS) || defined(_CCCL_DOXYGEN_INVOKED)
 
 #    define _CCCL_CONCEPT concept
 
@@ -225,7 +225,7 @@ namespace __unqualified_cuda_std = _CUDA_VSTD; // NOLINT(misc-unused-alias-decls
 //     );
 //
 // Can only be used as the last requirement in a concept definition.
-#  if defined(__cpp_concepts) && _CCCL_STD_VER >= 2020
+#  if !defined(_CCCL_NO_CONCEPTS) || defined(_CCCL_DOXYGEN_INVOKED)
 #    define _CCCL_REQUIRES_EXPR(_TY, ...) requires(__VA_ARGS__) _CCCL_REQUIRES_EXPR_2
 #    define _CCCL_REQUIRES_EXPR_2(...)    {_CCCL_PP_FOR_EACH(_CCCL_CONCEPT_FRAGMENT_REQS_M, __VA_ARGS__)}
 #  else