Merge expected-over-references: conformant std::expected implementation

Complete beman::expected implementation with all conformance fixes:
- expected<T,E> primary template
- expected<void,E> specialization
- unexpected<E>, bad_expected_access<E>
- Monadic operations (and_then, or_else, transform, transform_error)
- Trivial special member functions
- Hardened preconditions (BEMAN_EXPECTED_HARDENED)
- 253 tests passing

Author: steve-downey

.copier-answers.yml

@@ -6,7 +6,7 @@ maintainer: steve-downey
 minimum_cpp_build_version: '20'
 paper: PnnnnRr
 project_name: expected
-unit_test_library: gtest
+unit_test_library: catch2
 # Hidden variables manually tracked because 'when: false' omits them from _copier_answers
 generating_exemplar: false
 owner: "bemanproject"

.gitignore

@@ -21,3 +21,4 @@
 /.update-submodules
 /uv.lock
 .build
+/.claude/

CMakeLists.txt

@@ -82,11 +82,14 @@ beman_install_library(beman.expected TARGETS beman.expected)
 configure_build_telemetry()
 
 if(BEMAN_EXPECTED_BUILD_TESTS)
-    find_package(GTest CONFIG REQUIRED)
-endif()
-
-if(BEMAN_EXPECTED_BUILD_TESTS)
-    find_package(GTest CONFIG REQUIRED)
+    find_package(Catch2 CONFIG REQUIRED)
+    # When Catch2 is fetched via FetchContent, its extras dir needs to be on the module path.
+    # FetchContent_GetProperties reads global state set during fetch, so works from any scope.
+    include(FetchContent)
+    FetchContent_GetProperties(catch2)
+    if(catch2_POPULATED AND catch2_SOURCE_DIR)
+        list(APPEND CMAKE_MODULE_PATH ${catch2_SOURCE_DIR}/extras)
+    endif()
 endif()
 
 if(BEMAN_EXPECTED_BUILD_TESTS)

cmake/gcc-flags.cmake

@@ -1,8 +1,8 @@
 include_guard(GLOBAL)
 
-set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD 26)
 
-set(CMAKE_CXX_FLAGS "-Wall -Wextra -std=gnu++20" CACHE STRING "CXX_FLAGS" FORCE)
+set(CMAKE_CXX_FLAGS "-Wall -Wextra -std=gnu++26" CACHE STRING "CXX_FLAGS" FORCE)
 
 set(CMAKE_CXX_FLAGS_DEBUG
     "-O0 -fno-inline -g3"