Update: Using C++26 with contracts enabled

cmake/CourseOptions.cmake

@@ -21,7 +21,20 @@ function(course_apply_warnings target_name)
 endfunction()
 
 function(course_set_cxx_standard target_name standard)
-    target_compile_features(${target_name} PRIVATE cxx_std_${standard})
+    # Use explicit -std flags so older CLion/CMake builds still get C++26.
+    if(standard GREATER_EQUAL 26)
+        target_compile_options(${target_name} PRIVATE -std=c++26)
+    elseif(standard GREATER_EQUAL 23)
+        target_compile_options(${target_name} PRIVATE -std=c++23)
+    elseif(standard GREATER_EQUAL 20)
+        target_compile_options(${target_name} PRIVATE -std=c++20)
+    elseif(standard GREATER_EQUAL 17)
+        target_compile_options(${target_name} PRIVATE -std=c++17)
+    elseif(standard GREATER_EQUAL 14)
+        target_compile_options(${target_name} PRIVATE -std=c++14)
+    else()
+        target_compile_options(${target_name} PRIVATE -std=c++11)
+    endif()
 endfunction()
 
 function(course_enable_contracts target_name)

cmake/toolchains/gcc-16.cmake

@@ -1,3 +1,22 @@
-# Use GCC 16 for C++26 contracts support on macOS (Homebrew).
-set(CMAKE_C_COMPILER "/opt/homebrew/bin/gcc-16" CACHE FILEPATH "C compiler")
-set(CMAKE_CXX_COMPILER "/opt/homebrew/bin/g++-16" CACHE FILEPATH "C++ compiler")
+# GCC 16 toolchain for C++26 and contracts.
+find_program(
+    COURSE_GCC16_C
+    NAMES gcc-16
+    HINTS /opt/homebrew/bin /usr/local/bin /opt/local/bin
+)
+find_program(
+    COURSE_GCC16_CXX
+    NAMES g++-16
+    HINTS /opt/homebrew/bin /usr/local/bin /opt/local/bin
+)
+
+if(NOT COURSE_GCC16_CXX)
+    message(
+        FATAL_ERROR
+        "g++-16 not found. Install GCC 16, e.g. 'brew install gcc@16', "
+        "then ensure g++-16 is on your PATH."
+    )
+endif()
+
+set(CMAKE_C_COMPILER "${COURSE_GCC16_C}" CACHE FILEPATH "C compiler" FORCE)
+set(CMAKE_CXX_COMPILER "${COURSE_GCC16_CXX}" CACHE FILEPATH "C++ compiler" FORCE)