From 4ff04294c4abc150d8fc9377ef1b92686a996ef2 Mon Sep 17 00:00:00 2001
From: Denis Yaroshevskiy <dyaroshev@meta.com>
Date: Fri, 12 Jul 2024 03:40:18 -0700
Subject: [PATCH] folly concepts #1 (#2249)

Summary:
Pull Request resolved: https://github.com/facebook/folly/pull/2249

We have a need for some common concepts that are used in many places.

This is a proposal, please let me know what you think.

Differential Revision: D59396081
---
 folly/Portability.h            |  6 ++++++
 folly/Traits.h                 | 33 ++++++++++++++++++++++++++++++++
 folly/test/PortabilityTest.cpp |  8 ++++++--
 folly/test/TraitsTest.cpp      | 35 ++++++++++++++++++++++++++++++++++
 4 files changed, 80 insertions(+), 2 deletions(-)

diff --git a/folly/Portability.h b/folly/Portability.h
index 8334e2e99ab..8a99b035782 100644
--- a/folly/Portability.h
+++ b/folly/Portability.h
@@ -606,3 +606,9 @@ constexpr auto kCpplibVer = 0;
 #else
 #define FOLLY_CONSTEVAL constexpr
 #endif
+
+#if FOLLY_CPLUSPLUS >= 202002L && __has_include(<concepts>) && defined(__cpp_concepts)
+#define FOLLY_HAS_CONCEPTS 1
+#else
+#define FOLLY_HAS_CONCEPTS 0
+#endif
diff --git a/folly/Traits.h b/folly/Traits.h
index 001b45e7768..1eaea56c8be 100644
--- a/folly/Traits.h
+++ b/folly/Traits.h
@@ -25,6 +25,10 @@
 
 #include <folly/Portability.h>
 
+#if FOLLY_HAS_CONCEPTS
+#include <concepts>
+#endif
+
 namespace folly {
 
 #if defined(__cpp_lib_type_identity) && __cpp_lib_type_identity >= 201806L
@@ -168,6 +172,8 @@ namespace detail {
 /// Note that this only works with type template parameters. It does not work
 /// with non-type template parameters, template template parameters, or alias
 /// templates.
+///
+/// NOTE: there is also `instantiation_of` concept
 template <template <typename...> class, typename>
 inline constexpr bool is_instantiation_of_v = false;
 template <template <typename...> class C, typename... T>
@@ -186,6 +192,18 @@ struct is_similar_instantiation
 
 } // namespace detail
 
+#if FOLLY_HAS_CONCEPTS
+
+/**
+ * Concept to restrict a parameter to instantiation of a given template.
+ * NOTE: this only works with type template parameters.
+ */
+template <typename T, template <typename...> class Templ>
+concept instantiation_of =
+    detail::is_instantiation_of_v<Templ, std::remove_cvref_t<T>>;
+
+#endif
+
 /// member_pointer_traits
 ///
 /// For a member-pointer, reveals its constituent member-type and object-type.
@@ -1329,4 +1347,19 @@ template <typename T, typename... Dependencies>
 using enable_std_hash_helper =
     enable_hasher_helper<T, std::hash, Dependencies...>;
 
+#if FOLLY_HAS_CONCEPTS
+
+/**
+ *  Concept to restrict perfect forwarding to a specific type.
+ *
+ *  Example:
+ *  ```
+ *    void foo(folly::forwarded<std::vector<int>> auto&& vec);
+ *  ```
+ */
+template <typename Fwd, typename To>
+concept forwarded = std::same_as<std::remove_cvref_t<Fwd>, To>;
+
+#endif
+
 } // namespace folly
diff --git a/folly/test/PortabilityTest.cpp b/folly/test/PortabilityTest.cpp
index 2a8c17c70de..470e8a78033 100644
--- a/folly/test/PortabilityTest.cpp
+++ b/folly/test/PortabilityTest.cpp
@@ -25,6 +25,10 @@
 
 #include <folly/portability/GTest.h>
 
+#if FOLLY_HAS_CONCEPTS
+#include <concepts>
+#endif
+
 class Base {
  public:
   virtual ~Base() {}
@@ -52,6 +56,6 @@ TEST(Portability, Final) {
   EXPECT_EQ(3, fooDerived(p.get()));
 }
 
-#if __has_include(<range>)
-static_assert(std::ranges::random_access_range<std::vector<int>>);
+#if FOLLY_HAS_CONCEPTS
+static_assert(std::same_as<int, decltype(3)>);
 #endif
diff --git a/folly/test/TraitsTest.cpp b/folly/test/TraitsTest.cpp
index a5c02c101b2..c1c1c7d8c61 100644
--- a/folly/test/TraitsTest.cpp
+++ b/folly/test/TraitsTest.cpp
@@ -556,6 +556,21 @@ TEST(Traits, isInstantiationOf) {
   EXPECT_FALSE((detail::is_instantiation_of<A, B>::value));
 }
 
+TEST(Traits, InstantiationOf) {
+#if FOLLY_HAS_CONCEPTS
+  static_assert(folly::instantiation_of<A<int>, A>);
+  static_assert(folly::instantiation_of<A<int>&, A>);
+  static_assert(!folly::instantiation_of<A<int>&, std::vector>);
+
+  auto example = [](folly::instantiation_of<std::vector> auto&&) {};
+
+  static_assert(std::invocable<decltype(example), std::vector<int>&&>);
+  static_assert(std::invocable<decltype(example), std::vector<int>&>);
+  static_assert(std::invocable<decltype(example), const std::vector<int>&>);
+  static_assert(std::invocable<decltype(example), std::vector<int>>);
+#endif
+}
+
 TEST(Traits, isSimilarInstantiationV) {
   EXPECT_TRUE((detail::is_similar_instantiation_v<A<int>, A<long>>));
   EXPECT_FALSE((detail::is_similar_instantiation_v<A<int>, tag_t<int>>));
@@ -760,3 +775,23 @@ TEST(Traits, value_list) {
           folly::value_list_element_type_t<1, vtag_t<7u, 8, '9'>>>));
   EXPECT_EQ(8, (folly::value_list_element_v<1, vtag_t<7u, 8, '9'>>));
 }
+
+TEST(Traits, Forwarded) {
+#if FOLLY_HAS_CONCEPTS
+  static_assert(folly::forwarded<std::vector<int>, std::vector<int>>);
+  static_assert(folly::forwarded<std::vector<int>&, std::vector<int>>);
+  static_assert(folly::forwarded<const std::vector<int>&, std::vector<int>>);
+  static_assert(folly::forwarded<std::vector<int>&&, std::vector<int>>);
+
+  constexpr auto forwardedExample =
+      [](folly::forwarded<std::vector<int>> auto&&) {};
+
+  static_assert(std::invocable<decltype(forwardedExample), std::vector<int>>);
+  static_assert(
+      std::invocable<decltype(forwardedExample), const std::vector<int>&>);
+  static_assert(std::invocable<decltype(forwardedExample), std::vector<int>&&>);
+
+  static_assert(
+      !std::invocable<decltype(forwardedExample), std::vector<char>&&>);
+#endif
+}