Experiments with traits and concepts (#521)

exploration/BUILD.bazel

@@ -1,3 +1,5 @@
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
+
 cc_library(
     name = "wrapper",
     hdrs = ["wrapper.h"]
@@ -18,3 +20,14 @@ cc_test(
         "@com_google_googletest//:gtest_main",
     ]
 )
+
+cc_library(
+    name = "traits_and_concepts",
+    srcs = ["traits_and_concepts.cc"],
+    visibility = ["//visibility:private"],
+)
+
+build_test(
+    name = "traits_and_concepts_build_test",
+    targets = ["traits_and_concepts"],
+)

exploration/CMakeLists.txt

@@ -7,5 +7,17 @@ if (${XYZ_VALUE_TYPES_IS_NOT_SUBPROJECT})
                 incomplete_types.cc
                 incomplete_types_test.cc
         )
+
+        add_library(traits_and_concepts OBJECT)
+        target_sources(traits_and_concepts
+            PRIVATE
+                traits_and_concepts.cc
+        )
+        set_target_properties(traits_and_concepts PROPERTIES
+            CXX_STANDARD 20
+            CXX_STANDARD_REQUIRED YES
+            CXX_EXTENSIONS NO
+        )
+
     endif()
 endif()

exploration/traits_and_concepts.cc

@@ -0,0 +1,42 @@
+// An exploration of traits and concepts in C++ with incomplete types.
+
+#include <concepts>
+#include <map>
+#include <type_traits>
+
+namespace xyz::testing::traits_and_concepts {
+
+class Incomplete;
+
+// type-trait based checks will fail as type_traits cannot be evaluated on
+// incomplete types. AppleClang 16.0.0 (clang-1600.0.26.6):
+//  error: incomplete type 'xyz::testing::traits_and_concepts::Incomplete' used
+//  in type trait expression
+// static_assert(!std::is_copy_constructible_v<Incomplete>);
+// static_assert(std::is_copy_constructible_v<Incomplete>);
+
+// Concept check fails as AppleClang implements concept using type traits.
+// AppleClang 16.0.0 (clang-1600.0.26.6):
+//  error: incomplete type 'xyz::testing::traits_and_concepts::Incomplete' used
+//  in type trait expression
+// static_assert(!std::copy_constructible<Incomplete>);
+
+// Define our own concept to avoid restrictions from type traits.
+template <typename T>
+concept CopyConstructible = requires(const T& t) {
+  { T(t) } -> std::same_as<T>;
+};
+
+// The check below passes as the concept's requires clause is not satisfied for
+// an incomplete type.
+static_assert(!CopyConstructible<Incomplete>);
+
+class Incomplete {};
+
+// The check below will fail if CopyConstructible<Incomplete> was evaluated
+// above as the value of a concept does not change from one point of use to
+// another. If CopyConstructible<Incomplete> not was evaluated above, the check
+// below will pass as Incomplete is now complete and satisfies the concept
+// requirements. static_assert(CopyConstructible<Incomplete>);
+
+}  // namespace xyz::testing::traits_and_concepts