Update: Using C++26 with contracts enabled

README.md

@@ -17,37 +17,40 @@ brew install gcc@16
 
 ## Quick Start (CLion)
 
-1. Open this folder in CLion (`File → Open…`).
-2. Use the **`default`** CMake profile (`GCC 16 + C++26` from `CMakePresets.json`).
-3. Set the toolchain to **GCC 16** (`/opt/homebrew/bin/g++-16`).
-4. **Disable Clang-based analysis** (this project targets GCC only):
-   - **Settings → Languages & Frameworks → C/C++ → Clangd** → disable, **or**
-   - **Settings → Editor → Inspections → C/C++ → General → Clang-Tidy** → disable
-5. Reload CMake, pick an exercise target (e.g. `02_control_flow_loops`), implement TODOs, run tests.
+1. Open this folder in CLion.
+2. **Settings → Build, Execution, Deployment → CMake**
+3. Enable profile **`default`** (display name **GCC 16 + C++26** from `CMakePresets.json`).
+4. Disable old profiles that use Apple Clang (e.g. custom profiles without the preset toolchain).
+5. Click **Reload CMake Project**.
+
+If configure fails with *"This project requires GCC 16+"*, CLion is still using Apple Clang. Enable the **`default`** preset or set the toolchain C++ compiler to `g++-16`.
+
+If configure fails with *"generator does not match"*, delete the build folder (`cmake-build-debug-gcc-16-cpp26`) and reload CMake.
+
+6. Pick an exercise target (e.g. `02_control_flow_loops`) and run tests.
 
 ## Quick Start (Terminal)
 
 ```bash
 cmake --preset default
-cmake --build build
-ctest --test-dir build --output-on-failure
+cmake --build cmake-build-debug-gcc-16-cpp26
+ctest --test-dir cmake-build-debug-gcc-16-cpp26 --output-on-failure
 ```
 
 Run a single exercise:
 
 ```bash
-cmake --build build --target 02_control_flow_loops
-./build/modules/02_control_flow/02_control_flow_loops
+cmake --build cmake-build-debug-gcc-16-cpp26 --target 02_control_flow_loops
+./cmake-build-debug-gcc-16-cpp26/modules/02_control_flow/02_control_flow_loops
 ```
 
 Manual configure (without presets):
 
 ```bash
-cmake -S . -B build \
-  -DCMAKE_CXX_COMPILER=/opt/homebrew/bin/g++-16 \
-  -DCMAKE_C_COMPILER=/opt/homebrew/bin/gcc-16 \
+cmake -S . -B cmake-build-debug-gcc-16-cpp26 \
+  -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/gcc-16.cmake \
   -DCOURSE_ENABLE_CONTRACTS=ON
-cmake --build build
+cmake --build cmake-build-debug-gcc-16-cpp26
 ```
 
 ## Project Structure
@@ -143,10 +146,43 @@ auto factorial(const int n) -> long long
 
 - Use the **GCC 16** toolchain, not Apple Clang.
 - Disable **Clangd** and **Clang-Tidy** inspections — they use the Clang frontend and do not support GCC C++26 contracts.
-- Use CLion’s built-in **GCC** parser/inspections, or rely on compiler errors from the build.
+- **Cppcheck plugin:** disable inline analysis for this project (see [Cppcheck](#cppcheck) below).
 - Use **Run | Run…** and filter by module prefix (e.g. `02_control_flow`).
 - Enable **Google Test** integration to see individual tests in the tree view.
 
+## Cppcheck
+
+Cppcheck **does not understand GCC C++26 contracts** (`pre(...)`) or GCC 16 libstdc++ when the CLion plugin analyzes headers in isolation. That produces false IDE errors such as:
+
+- `Include file: <contracts> not found`
+- `failed to evaluate #elif condition` in libstdc++ headers
+- parse errors on `pre(...)`
+
+### Recommended: disable the plugin, use the CMake target
+
+1. **Settings → Editor → Inspections → C/C++ → Cppcheck** → **disable**
+2. Configure and build once:
+   ```bash
+   cmake --preset default
+   cmake --build build
+   ```
+3. Run Cppcheck from CLion or terminal:
+   ```bash
+   cmake --build build --target cppcheck_modules
+   ```
+
+This uses `compile_commands.json` from your **GCC 16** build and suppresses known false positives (`cppcheck/suppressions.txt`).
+
+### If you keep the plugin anyway
+
+**Settings → Cppcheck configuration → Options:**
+
+```
+--language=c++ --std=c++26 --max-configs=1 --suppress=missingIncludeSystem --suppress=syntaxError
+```
+
+Expect remaining false positives on contract syntax. The plugin is not a good fit for this codebase.
+
 ## Notes on C++26
 
 All exercises build with **C++26**. Module READMEs describe the language feature each exercise teaches (C++11, C++17, etc.), but the toolchain baseline is C++26 throughout.