Update: Using C++26 with contracts enabled

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)