c++: add concept (c++20) example

3 files changed, 115 insertions, 2 deletions

diff --git a/src/development/c++.md b/src/development/c++.md
index 2df60c2..1965e33 100644
--- a/src/development/c++.md
+++ b/src/development/c++.md
@@ -283,12 +283,17 @@ specialization for `is_valid` will be removed from the candidate set due to
 A more detailed description is available in the SO discussion [How does
 `void_t` work](https://stackoverflow.com/a/27688405).
 
+## Example: Concepts since c++20
+```cpp
+{{#include c++/concepts-20.cc:4:}}
+```
+
 ## Template selection with partially / fully specializations.
 ```cpp
 {{#include c++/tmpl-pair.cc:3:}}
 ```
 
-# Example: Perfect forwarding
+## Example: Perfect forwarding
 ```cpp
 {{#include c++/fwd-perfect.cc:3:}}
 ```
diff --git a/src/development/c++/Makefile b/src/development/c++/Makefile
index 622bfd3..2c53e47 100644
--- a/src/development/c++/Makefile
+++ b/src/development/c++/Makefile
@@ -1,4 +1,4 @@
-SRC = concepts-11.cc meta.cc meta2.cc meta4.cc tmpl-pair.cc tmpl-void_t.cc fwd.cc fwd-perfect.cc
+SRC = concepts-11.cc concepts-20.cc meta.cc meta2.cc meta4.cc tmpl-pair.cc tmpl-void_t.cc fwd.cc fwd-perfect.cc
 BIN = $(SRC:.cc=)
 
 all: $(BIN)
@@ -6,6 +6,9 @@ all: $(BIN)
 %: %.cc
 	$(CXX) -o $* $^ -std=c++17 -g -fsanitize=address -fsanitize=undefined -fsanitize=leak
 
+concepts-20: concepts-20.cc
+	$(CXX) -fsyntax-only $^ -std=c++20
+
 fmt:
 	clang-format -i $(SRC)
 
diff --git a/src/development/c++/concepts-20.cc b/src/development/c++/concepts-20.cc
new file mode 100644
index 0000000..018b30b
--- /dev/null
+++ b/src/development/c++/concepts-20.cc
@@ -0,0 +1,105 @@
+#include <concepts>
+#include <type_traits>
+
+// REQUIRES EXPRESSION
+//   requires { requirement-seq }
+//   requires ( parameter-list ) { requirement-seq }
+//
+// [1] https://en.cppreference.com/w/cpp/language/requires
+// [2] https://en.cppreference.com/w/cpp/language/constraints#Constraints
+//
+// REQUIREMENT CLAUSE
+//   Not the same as a REQUIREMENT EXPRESSIONS, and is used to require
+//   constraints (express concept bounds).
+//
+// [1] https://en.cppreference.com/w/cpp/language/constraints#Requires_clauses
+
+// -- HELPER -------------------------------------------------------------------
+
+template<typename T>
+using Alias = T;
+
+void print(int);
+
+// -- CONCEPTS & REQUIRE EXPRESSIONS -------------------------------------------
+
+// Simple concept from a type trait.
+template<typename T, typename U>
+concept Same = std::is_same<T, U>::value;
+
+// Simple requirement concepts.
+template<typename T>
+concept TraitAddAndPrint = requires(T t, int i) {
+    // Adding T + int must be supported.
+    t + i;
+    // Calling print(T) must be available.
+    print(t);
+};
+
+// Type requirement concepts.
+template<typename T>
+concept TraitTypes = requires(T t) {
+    // T must have a type definition inner.
+    typename T::inner;
+    // Type alias must exist.
+    typename Alias<T>;
+};
+
+// Compound requirement concepts.
+template<typename T>
+concept TraitFns = requires(T t, const T c) {
+    // void T::foo() must exist.
+    { t.foo() };
+    // bool T::bar() const; must exist.
+    { c.bar() } -> Same<bool>;
+    // static void T::stat(); must exist.
+    { T::stat() } -> Same<int>;
+};
+
+// Nested requirement concepts.
+template<typename T>
+concept TraitNested = requires(T t) {
+    // Must satisfy other concepts.
+    requires TraitTypes<T>;
+    requires TraitFns<T>;
+};
+
+// -- REQUIRE EXPRESSIONS ------------------------------------------------------
+
+// Require expressions can be evaluated to booleans.
+template<typename T>
+static constexpr bool IsTraitFns = requires { requires TraitFns<T>; };
+
+// Require expressions can also be used in static assertions.
+static_assert(requires { requires Same<int, int>; });
+static_assert(!requires {
+    typename Alias<int>;
+    requires Same<int, void>;
+});
+
+// -- TESTS --------------------------------------------------------------------
+
+static_assert(requires { requires TraitAddAndPrint<int>; });
+
+struct FnTypeGood {
+    using inner = int;
+};
+struct FnTypeBad {};
+static_assert(requires { requires TraitTypes<FnTypeGood>; });
+static_assert(!requires { requires TraitTypes<FnTypeBad>; });
+
+struct FnGood {
+    void foo();
+    bool bar() const;
+    static int stat();
+};
+struct FnBad {};
+static_assert(requires { requires TraitFns<FnGood>; });
+static_assert(!requires { requires TraitFns<FnBad>; });
+
+struct NestedGood : FnTypeGood, FnGood {};
+struct NestedBad1 : FnGood {};
+struct NestedBad2 : FnTypeGood {};
+static_assert(requires { requires TraitNested<NestedGood>; });
+static_assert(!requires { requires TraitNested<NestedBad1>; });
+static_assert(!requires { requires TraitNested<NestedBad2>; });