[Backend] Establish standalone build infrastructure

Add the thin monorepo CMake entry point and an independently configurable backend project. Enforce the Clang/C++23 toolchain, expose Make/Ninja-neutral tests and developer checks, export an installable RCCBackend package, and introduce the first Support error abstraction.

3 files changed, 52 insertions, 0 deletions

diff --git a/backend/unittests/CMakeLists.txt b/backend/unittests/CMakeLists.txt
new file mode 100644
index 0000000..13f7c7b
--- /dev/null
+++ b/backend/unittests/CMakeLists.txt
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+add_subdirectory(Support)
diff --git a/backend/unittests/Support/CMakeLists.txt b/backend/unittests/Support/CMakeLists.txt
new file mode 100644
index 0000000..0c0aabc
--- /dev/null
+++ b/backend/unittests/Support/CMakeLists.txt
@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+set(RCCBackendSupportTestSources ErrorTest.cpp)
+
+add_executable(RCCBackendSupportTests ${RCCBackendSupportTestSources})
+rcc_backend_configure_target(RCCBackendSupportTests)
+target_link_libraries(RCCBackendSupportTests PRIVATE RCCBackend::Support)
+
+add_test(NAME RCCBackend.Support.Error COMMAND RCCBackendSupportTests)
+
+rcc_backend_register_format_files(
+  "${CMAKE_CURRENT_SOURCE_DIR}/ErrorTest.cpp"
+)
+rcc_backend_register_tidy_files(
+  "${CMAKE_CURRENT_SOURCE_DIR}/ErrorTest.cpp"
+)
diff --git a/backend/unittests/Support/ErrorTest.cpp b/backend/unittests/Support/ErrorTest.cpp
new file mode 100644
index 0000000..23140e3
--- /dev/null
+++ b/backend/unittests/Support/ErrorTest.cpp
@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include "rcc/Support/Error.h"
+
+#include <iostream>
+#include <string>
+
+namespace {
+
+int fail(const char *Message) {
+  std::cerr << Message << '\n';
+  return 1;
+}
+
+} // namespace
+
+int main() {
+  rcc::Expected<int> Value = 42;
+  if (!Value || *Value != 42)
+    return fail("Expected must preserve a successful value");
+
+  rcc::Expected<int> Failure = rcc::makeUnexpected(rcc::ErrorCode::ParseError, "bad MIR");
+  if (Failure)
+    return fail("Expected must preserve an error");
+  if (Failure.error().code() != rcc::ErrorCode::ParseError)
+    return fail("Error must preserve its code");
+  if (Failure.error().message() != "bad MIR")
+    return fail("Error must preserve its message");
+  if (rcc::getErrorCodeName(Failure.error().code()) != "parse error")
+    return fail("Error code must have a stable name");
+
+  return 0;
+}