Add type_traits unit test

tests/src/unit-type_traits.cpp

@@ -0,0 +1,56 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.11.1
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2022 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#if JSON_TEST_USING_MULTIPLE_HEADERS
+    #include <nlohmann/detail/meta/type_traits.hpp>
+#else
+    #include <nlohmann/json.hpp>
+#endif
+
+TEST_CASE("type traits")
+{
+    SECTION("is_c_string")
+    {
+        using nlohmann::detail::is_c_string;
+        using nlohmann::detail::is_c_string_uncvref;
+
+        SECTION("char *")
+        {
+            CHECK(is_c_string<char*>::value);
+            CHECK(is_c_string<const char*>::value);
+            CHECK(is_c_string<char* const>::value);
+            CHECK(is_c_string<const char* const>::value);
+
+            CHECK_FALSE(is_c_string<char*&>::value);
+            CHECK_FALSE(is_c_string<const char*&>::value);
+            CHECK_FALSE(is_c_string<char* const&>::value);
+            CHECK_FALSE(is_c_string<const char* const&>::value);
+
+            CHECK(is_c_string_uncvref<char*&>::value);
+            CHECK(is_c_string_uncvref<const char*&>::value);
+            CHECK(is_c_string_uncvref<char* const&>::value);
+            CHECK(is_c_string_uncvref<const char* const&>::value);
+        }
+
+        SECTION("char[]")
+        {
+            // NOLINTBEGIN(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+            CHECK(is_c_string<char[]>::value);
+            CHECK(is_c_string<const char[]>::value);
+
+            CHECK_FALSE(is_c_string<char(&)[]>::value);
+            CHECK_FALSE(is_c_string<const char(&)[]>::value);
+
+            CHECK(is_c_string_uncvref<char(&)[]>::value);
+            CHECK(is_c_string_uncvref<const char(&)[]>::value);
+            // NOLINTEND(hicpp-avoid-c-arrays,modernize-avoid-c-arrays,cppcoreguidelines-avoid-c-arrays)
+        }
+    }
+}