cpp(placeholders): fix the static analysis

Signed-off-by: Matej Focko <mfocko@redhat.com>

cpp/07-exceptions-and-raii/2023-11-24-placeholders.md

@@ -203,3 +203,20 @@ void unreachable(std::format_string<Args...> fmt, Args&&... args) {
 Final source code: [`placeholders.hpp`](pathname:///files/cpp/exceptions-and-raii/placeholders/placeholders.hpp)
 
 :::
+
+## Post-mortem
+
+One of the things, I've forgotten about, is the fact that static analysis of
+your code has no way to know those helper functions we've created as shortcuts
+don't return and just throw the exception right away. Therefore we need to mark
+them with `[[noreturn]]` to let the static analysis know that we **never**
+return from such functions. For example:
+
+```cpp
+[[noreturn]] void unreachable() { throw _unreachable(); }
+
+template <class... Args>
+[[noreturn]] void unreachable(std::format_string<Args...> fmt, Args&&... args) {
+    throw _unreachable(std::format(fmt, args...));
+}
+```

static/files/cpp/exceptions-and-raii/placeholders/placeholders.hpp

@@ -14,7 +14,7 @@ class _todo : public std::exception {
 /**
  * @brief Indicates unfinished code.
  */
-void todo() { throw _todo(); }
+[[noreturn]] void todo() { throw _todo(); }
 
 /**
  * @brief Indicates unfinished code.
@@ -22,7 +22,7 @@ void todo() { throw _todo(); }
  * @param args arguments to be formatted
  */
 template <class... Args>
-void todo(std::format_string<Args...> fmt, Args&&... args) {
+[[noreturn]] void todo(std::format_string<Args...> fmt, Args&&... args) {
     throw _todo(std::format(fmt, args...));
 }
 
@@ -39,7 +39,7 @@ class _unimplemented : public std::exception {
 /**
  * @brief Indicates unimplemented code by throwing with a message of “not implemented”.
  */
-void unimplemented() { throw _unimplemented(); }
+[[noreturn]] void unimplemented() { throw _unimplemented(); }
 
 /**
  * @brief Indicates unimplemented code by throwing with a message of “not implemented”.
@@ -47,7 +47,7 @@ void unimplemented() { throw _unimplemented(); }
  * @param args arguments to be formatted
  */
 template <class... Args>
-void unimplemented(std::format_string<Args...> fmt, Args&&... args) {
+[[noreturn]] void unimplemented(std::format_string<Args...> fmt, Args&&... args) {
     throw _unimplemented(std::format(fmt, args...));
 }
 
@@ -64,7 +64,7 @@ class _unreachable : public std::exception {
 /**
  * @brief Indicates unreachable code.
  */
-void unreachable() { throw _unreachable(); }
+[[noreturn]] void unreachable() { throw _unreachable(); }
 
 /**
  * @brief Indicates unreachable code.
@@ -72,6 +72,6 @@ void unreachable() { throw _unreachable(); }
  * @param args arguments to be formatted
  */
 template <class... Args>
-void unreachable(std::format_string<Args...> fmt, Args&&... args) {
+[[noreturn]] void unreachable(std::format_string<Args...> fmt, Args&&... args) {
     throw _unreachable(std::format(fmt, args...));
 }