4 files changed, 279 insertions, 1 deletions
diff --git a/libs/kstd/CMakeLists.txt b/libs/kstd/CMakeLists.txt
index 1902d0d9..d7af1952 100644
--- a/libs/kstd/CMakeLists.txt
+++ b/libs/kstd/CMakeLists.txt
@@ -83,6 +83,10 @@ if(BUILD_TESTING)
 
     teachos_add_tests("kstd")
 
+    target_compile_options("kstd_tests" PRIVATE
+        "SHELL:-include ${CMAKE_CURRENT_SOURCE_DIR}/kstd/test_support/string_makers.hpp"
+    )
+
     if(COMMAND "enable_coverage" AND KSTD_ENABLE_TEST_COVERAGE)
         enable_coverage("kstd")
         enable_coverage("kstd_tests")
diff --git a/libs/kstd/kstd/test_support/os_panic.tests.cpp b/libs/kstd/kstd/test_support/os_panic.tests.cpp
index c30411aa..17af74a7 100644
--- a/libs/kstd/kstd/test_support/os_panic.tests.cpp
+++ b/libs/kstd/kstd/test_support/os_panic.tests.cpp
@@ -7,7 +7,7 @@
 namespace kstd::os
 {
 
-  auto panic(std::string_view message, std::source_location location)
+  auto panic(std::string_view message, std::source_location location) -> void
   {
     throw kstd::tests::os_panic{std::string{message}, location};
   }
diff --git a/libs/kstd/kstd/test_support/string_makers.hpp b/libs/kstd/kstd/test_support/string_makers.hpp
new file mode 100644
index 00000000..c2c6a38a
--- /dev/null
+++ b/libs/kstd/kstd/test_support/string_makers.hpp
@@ -0,0 +1,180 @@
+#ifndef KSTD_TEST_SUPPORT_STRING_MAKERS_HPP
+#define KSTD_TEST_SUPPORT_STRING_MAKERS_HPP
+
+//! Catch2 string conversions for kstd types.
+//!
+//! Catch2 prints values it does not know how to convert as "{?}". This header teaches Catch2 how to print kstd types,
+//! so that failed assertions show the actual values. It is force-included into every test translation unit by the
+//! build system and must therefore never be included by kernel code.
+//!
+//! Any type with a kstd::formatter specialization is printed via kstd::format, so the easiest way to make a type
+//! printable in tests is to give it a formatter. Types without a formatter can get a Catch::StringMaker specialization
+//! here, or in the test support header of the library they belong to.
+
+#include <kstd/format.hpp>
+#include <kstd/memory.hpp>
+#include <kstd/string.hpp>
+#include <kstd/system_error.hpp>
+
+#include <catch2/catch_tostring.hpp>
+
+#include <concepts>
+#include <expected>
+#include <ostream>
+#include <ranges>
+#include <string>
+#include <type_traits>
+
+namespace kstd::tests
+{
+
+  //! A type that can be printed via kstd::format.
+  template<typename T>
+  concept kstd_formattable = std::default_initializable<kstd::formatter<std::remove_cvref_t<T>>>;
+
+  //! A type that can be written to a std::ostream.
+  template<typename T>
+  concept stream_insertable = requires(std::ostream & stream, T const & value) { stream << value; };
+
+  //! A type that Catch2 can not print on its own, but which is printable via kstd::format.
+  //!
+  //! Types that are stream insertable, pointers, arrays, and ranges are handled by Catch2 already. They are excluded
+  //! before checking for a formatter, since probing the formatter of a range with unformattable elements is a hard
+  //! error.
+  template<typename T>
+  concept catch_printable_via_kstd =
+      !std::ranges::range<T> && !Catch::is_range<T>::value && !std::is_pointer_v<T> && !std::is_array_v<T> &&
+      !std::is_arithmetic_v<T> && !stream_insertable<T> && kstd_formattable<T>;
+
+  //! Convert a kstd string to a std::string.
+  inline auto to_std_string(kstd::string const & string) -> std::string
+  {
+    return std::string{string.data(), string.size()};
+  }
+
+  //! Convert an error code or error condition to its textual representation, e.g. "generic:22:invalid argument".
+  template<typename ErrorType>
+  auto error_to_string(ErrorType const & error) -> std::string
+  {
+    return std::string{error.category().name()} + ':' + std::to_string(error.value()) + ':' +
+           std::string{error.message()};
+  }
+
+  //! Convert a smart pointer to the same textual representation Catch2 uses for raw pointers.
+  template<typename Pointer>
+  auto pointer_to_string(Pointer const & pointer) -> std::string
+  {
+    return Catch::Detail::stringify(pointer.get());
+  }
+
+}  // namespace kstd::tests
+
+namespace Catch
+{
+
+  template<kstd::tests::catch_printable_via_kstd T>
+  struct StringMaker<T>
+  {
+    static auto convert(T const & value) -> std::string
+    {
+      return kstd::tests::to_std_string(kstd::format("{}", value));
+    }
+  };
+
+  template<>
+  struct StringMaker<kstd::string>
+  {
+    static auto convert(kstd::string const & value) -> std::string
+    {
+      return Catch::Detail::stringify(kstd::tests::to_std_string(value));
+    }
+  };
+
+  template<>
+  struct StringMaker<kstd::error_category>
+  {
+    static auto convert(kstd::error_category const & category) -> std::string
+    {
+      return std::string{category.name()};
+    }
+  };
+
+  template<>
+  struct StringMaker<kstd::error_condition>
+  {
+    static auto convert(kstd::error_condition const & condition) -> std::string
+    {
+      return kstd::tests::error_to_string(condition);
+    }
+  };
+
+  template<kstd::error_code_enum Enum>
+  struct StringMaker<Enum>
+  {
+    static auto convert(Enum value) -> std::string
+    {
+      return kstd::tests::error_to_string(kstd::error_code{value});
+    }
+  };
+
+  template<kstd::error_condition_enum Enum>
+    requires(!kstd::error_code_enum<Enum>)
+  struct StringMaker<Enum>
+  {
+    static auto convert(Enum value) -> std::string
+    {
+      return kstd::tests::error_to_string(make_error_condition(value));
+    }
+  };
+
+  template<typename T>
+  struct StringMaker<kstd::shared_ptr<T>>
+  {
+    static auto convert(kstd::shared_ptr<T> const & pointer) -> std::string
+    {
+      return kstd::tests::pointer_to_string(pointer);
+    }
+  };
+
+  template<typename T>
+  struct StringMaker<kstd::unique_ptr<T>>
+  {
+    static auto convert(kstd::unique_ptr<T> const & pointer) -> std::string
+    {
+      return kstd::tests::pointer_to_string(pointer);
+    }
+  };
+
+  template<typename T>
+  struct StringMaker<kstd::observer_ptr<T>>
+  {
+    static auto convert(kstd::observer_ptr<T> const & pointer) -> std::string
+    {
+      return kstd::tests::pointer_to_string(pointer);
+    }
+  };
+
+  template<typename T, typename E>
+  struct StringMaker<std::expected<T, E>>
+  {
+    static auto convert(std::expected<T, E> const & expected) -> std::string
+    {
+      if (!expected)
+      {
+        return "error(" + Catch::Detail::stringify(expected.error()) + ')';
+      }
+
+      if constexpr (std::is_void_v<T>)
+      {
+        return "value()";
+      }
+      else
+      {
+        return "value(" + Catch::Detail::stringify(*expected) + ')';
+      }
+    }
+  };
+
+}  // namespace Catch
+
+#endif
diff --git a/libs/kstd/kstd/test_support/string_makers.tests.cpp b/libs/kstd/kstd/test_support/string_makers.tests.cpp
new file mode 100644
index 00000000..887bec37
--- /dev/null
+++ b/libs/kstd/kstd/test_support/string_makers.tests.cpp
@@ -0,0 +1,94 @@
+#include <kstd/test_support/string_makers.hpp>
+
+#include <kstd/memory.hpp>
+#include <kstd/result.hpp>
+#include <kstd/string.hpp>
+#include <kstd/system_error.hpp>
+#include <kstd/units.hpp>
+
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/catch_tostring.hpp>
+
+#include <compare>
+#include <expected>
+
+using namespace kstd::units_literals;
+
+SCENARIO("Catch2 string conversion of kstd types", "[test_support][string_makers]")
+{
+  GIVEN("a kstd string")
+  {
+    auto const string = kstd::string{"hello"};
+
+    THEN("it is printed like a std::string")
+    {
+      REQUIRE(Catch::Detail::stringify(string) == "\"hello\"");
+    }
+  }
+
+  GIVEN("a type with a kstd formatter")
+  {
+    THEN("it is printed via kstd::format")
+    {
+      REQUIRE(Catch::Detail::stringify(42_B) == "42");
+      REQUIRE(Catch::Detail::stringify(std::strong_ordering::less) == "less");
+    }
+  }
+
+  GIVEN("an error code")
+  {
+    auto const error = kstd::make_error_code(kstd::errc::invalid_argument);
+
+    THEN("it is printed with its category, value, and message")
+    {
+      REQUIRE(Catch::Detail::stringify(error) == kstd::tests::error_to_string(error));
+      REQUIRE(Catch::Detail::stringify(error).starts_with(std::string{error.category().name()} + ':'));
+    }
+  }
+
+  GIVEN("an error condition enumerator")
+  {
+    THEN("it is printed like the matching error condition")
+    {
+      REQUIRE(Catch::Detail::stringify(kstd::errc::invalid_argument) ==
+              kstd::tests::error_to_string(kstd::make_error_condition(kstd::errc::invalid_argument)));
+    }
+  }
+
+  GIVEN("a successful result")
+  {
+    auto const result = kstd::result<int>{42};
+    auto const void_result = kstd::result<void>{};
+
+    THEN("it is printed as value(...)")
+    {
+      REQUIRE(Catch::Detail::stringify(result) == "value(42)");
+      REQUIRE(Catch::Detail::stringify(void_result) == "value()");
+    }
+  }
+
+  GIVEN("a failed result")
+  {
+    auto const error = kstd::make_error_code(kstd::errc::invalid_argument);
+    auto const result = kstd::result<int>{std::unexpect, error};
+
+    THEN("it is printed as error(...)")
+    {
+      REQUIRE(Catch::Detail::stringify(result) == "error(" + Catch::Detail::stringify(error) + ')');
+    }
+  }
+
+  GIVEN("smart pointers")
+  {
+    auto const empty = kstd::shared_ptr<int>{};
+    auto const shared = kstd::make_shared<int>(1);
+    auto const observer = kstd::observer_ptr<int>{shared.get()};
+
+    THEN("they are printed like raw pointers")
+    {
+      REQUIRE(Catch::Detail::stringify(empty) == "nullptr");
+      REQUIRE(Catch::Detail::stringify(shared) == Catch::Detail::stringify(shared.get()));
+      REQUIRE(Catch::Detail::stringify(observer) == Catch::Detail::stringify(shared.get()));
+    }
+  }
+}