On: ISO C became unusable for operating systems develo...

[mjevans]

How about circumstances such as opting in to a semantically new version of C?

  #ifndef __CC_VOLATILE_FUNCTIONS
  /\* No volatile function support, remove code */
  #define VOLFUNC
  #else

/* This compiler supports volatile functions. Only volatile functions may cause external side effects without likely bugs. */

  #define VOLFUNC volatile
  #endif

https://www.postgresql.org/docs/current/xfunc-volatility.htm...

Similarly stable functions could have results cached. You might also note that PostgreSQL assumes any undeclared function is volatile.