add some tests for concepts

CMakeLists.txt

@@ -32,3 +32,4 @@ enable_testing()
 find_package(GTest REQUIRED)
 
 add_subdirectory(cpp20_modules)
+add_subdirectory(cpp20_concepts)

cpp20_concepts/CMakeLists.txt

@@ -0,0 +1,2 @@
+add_subdirectory(lib)
+add_subdirectory(test)

cpp20_concepts/lib/CMakeLists.txt

@@ -0,0 +1,8 @@
+set(TARGET concept_lib)
+add_library(${TARGET})
+set_target_properties(${TARGET} PROPERTIES LINKER_LANGUAGE CXX)
+
+set(SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/include/hashable.h)
+
+target_sources(${TARGET} PUBLIC ${SOURCES})
+target_include_directories(${TARGET} PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include)

cpp20_concepts/lib/include/hashable.h

@@ -0,0 +1,14 @@
+// Declaration of the concept "Hashable", which is satisfied by any type 'T'
+// such that for values 'a' of type 'T', the expression std::hash<T>{}(a)
+// compiles and its result is convertible to std::size_t
+#include <functional>
+
+template <typename T>
+concept Hashable = requires(T a)
+{
+    {
+        std::hash<T>{}(a)
+        } -> std::convertible_to<std::size_t>;
+};
+
+// https://en.cppreference.com/w/cpp/language/constraints

cpp20_concepts/test/CMakeLists.txt

@@ -0,0 +1,7 @@
+set(TARGET concept_tests)
+
+add_executable(${TARGET})
+target_sources(${TARGET} PRIVATE tests.cpp)
+target_link_libraries(${TARGET} PRIVATE concept_lib GTest::gtest
+                                        GTest::gtest_main)
+add_test(NAME ${TARGET} COMMAND ${TARGET})

cpp20_concepts/test/tests.cpp

@@ -0,0 +1,25 @@
+#include <gtest/gtest.h>
+
+#include <hashable.h>
+
+struct meow
+{
+};
+
+// Constrained C++20 function template:
+template <Hashable T>
+T hashable_func1(T t) { return t; }
+
+// Alternative ways to apply the same constraint:
+template <typename T>
+requires Hashable<T>
+    T hashable_func2(T t) { return t; }
+
+template <typename T>
+T hashable_func3(T t) requires Hashable<T> { return t; }
+
+int main()
+{
+    auto const result = hashable_func1(1) + hashable_func2(2) + hashable_func3(3);
+    return !(6ULL == result);
+}