Add compile-time-settings-check.md

cpp/compile-time-settings-check.md

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+It is sometimes useful to be able to verify that certain settings are valid, at
+compile time. An easy approach to do this is to pass the settings as non-type
+template parameters and use `static_assert()`.
+
+The code snippet below demonstrates an approach which doesn't require templates
+and cleanly separates the verification logic from the rest of the code.
+
+```cpp
+namespace i2c {
+
+
+//
+//
+// Driver Settings
+//
+//
+
+
+struct Settings {
+    int         sclPin = 0;
+    int         sdaPin = 0;
+    std::size_t freqHz = 1'000'000;
+};
+
+
+namespace detail {
+struct SettingsChecker {
+public:
+    consteval SettingsChecker(Settings settings) : mSettings{settings} {
+        bool validPinSettings =
+            (settings.sclPin == 1 && settings.sdaPin == 2) ||
+            (settings.sclPin == 3 && settings.sdaPin == 4);
+        bool validFreqSettings =
+            (settings.freqHz < 10'000'000) && (settings.freqHz > 100);
+
+        // Exceptions thrown in consteval functions lead to compile time errors
+
+        if (!validPinSettings) throw std::logic_error("Wrong I2C pin settings");
+        if (!validFreqSettings)
+            throw std::logic_error("Wrong I2C frequency settings");
+    }
+
+    constexpr operator Settings() const {
+        return mSettings;
+    }
+
+private:
+    Settings mSettings;
+};
+} // namespace detail
+
+
+//
+//
+// Driver class
+//
+//
+
+
+class Driver {
+public:
+    Driver(detail::SettingsChecker settings) : mSettings(settings) {
+    }
+
+
+private:
+    Settings mSettings;
+};
+
+
+} // namespace i2c
+
+
+//
+//
+// Usage
+//
+//
+
+
+int main() {
+    constexpr i2c::Settings settings1 = {
+        .sclPin = 1, .sdaPin = 2, .freqHz = 1'000'000};
+
+    i2c::Driver driver1{settings1}; // Compiles
+
+    constexpr i2c::Settings settings2 = {
+        .sclPin = 1, .sdaPin = 4, .freqHz = 1'000'000};
+
+    i2c::Driver driver2{settings2}; // Compile time error
+}
+```
+
+Compiling the above code leads to the following error:
+
+```bash
+
+compile-time-settings-check.cpp: In function ‘int main()’:
+compile-time-settings-check.cpp:88:34: error: call to consteval function ‘i2c::detail::SettingsChecker(settings2)’ is not a constant expression
+   88 |     i2c::Driver driver2{settings2}; // Compile-time error
+      |                                  ^
+compile-time-settings-check.cpp:88:34:   in ‘constexpr’ expansion of ‘i2c::detail::SettingsChecker(settings2)’
+compile-time-settings-check.cpp:36:32: error: expression ‘<throw-expression>’ is not a constant expression
+   36 |         if (!validPinSettings) throw std::logic_error("Wrong I2C pin settings");
+      |
+```