Added initial support for C++20 concepts

tests/t00056/t00056.cc

@@ -1,10 +1,57 @@
+#include <string>
+
 namespace clanguml {
 namespace t00056 {
+// Constraint expression
 template <typename T>
 concept MaxFourBytes = sizeof(T) <= 4;
 
+// Simple requirement
+template <typename T>
+concept Iterable = requires(T container) {
+                       container.begin();
+                       container.end();
+                   };
+
+// Type requirement
+template <typename T>
+concept HasValueType = requires { typename T::value_type; };
+
+template <typename T>
+concept ConvertibleToString = requires(T s) { std::string{s}; };
+
+// Compound requirement
+// ...
+
+// Combined concept
+template <typename T>
+concept IterableWithValueType = Iterable<T> && HasValueType<T>;
+
+template <typename T>
+concept IterableWithSmallValueType =
+    IterableWithValueType<T> && MaxFourBytes<T>;
+
+// Simple type constraint
 template <MaxFourBytes T> struct A {
     T a;
 };
+
+// Requires constant expression
+template <typename T>
+    requires MaxFourBytes<T>
+struct B {
+    T b;
+};
+
+// Anonymous concept requirement
+template <typename T>
+    requires requires(T t) {
+                 --t;
+                 t--;
+             }
+struct C {
+    T c;
+};
+
 }
 }