-/// Prints the message and location info to stderr in !NDEBUG builds.
-/// This is intended to be used for "impossible" situations that imply
-/// a bug in the compiler.
+/// Marks that the current location is not supposed to be reachable.
+/// In !NDEBUG builds, prints the message and location info to stderr.
+/// In NDEBUG builds, becomes an optimizer hint that the current location
+/// is not supposed to be reachable. On compilers that don't support
+/// such hints, prints a reduced message instead.