better type safety and type-related error reporting in C89 macros .c

/* two abi-equivalent, but nominatively distinct structs */

struct A {
  int a;
  int b;
};

struct B {
  int a;
  int b;
};

/* what if you want the following macro to fail with an error message about "incompatible types" if you pass in a B instead of an A upon expansion? */

#define SOME_MACRO(SOME_STRUCT) do {\
  (SOME_STRUCT)->a = 100;\
  (SOME_STRUCT)->b = 200;\
} while (0)

/* ...you do this */

extern int _ensure_is_struct_A(struct A _);

#define SOME_MACRO_TYPE_SAFE(SOME_STRUCT) do {\
  /* how to write an empty statement in C89 that doesn't emit warnings: (void) <some expression>; */
  /* also, did you know? the operand of sizeof is not evaluated unless it's a VLA (a C99-onwards feature), and because it's not evaluated, the linker doesn't trip up looking for the implementation of _ensure_is_struct_A(). */
  /* and voila, some level of type safety in macros without blowing up your linter, pragma-free! */
  (void) sizeof (_ensure_is_struct_A(*(SOME_STRUCT)));\
  SOME_STRUCT->a = 100;\
  SOME_STRUCT->b = 100;\
} while (0)

/* in some .c file: */

/* ... */

struct B i_am_a_b_struct;

/* when this macro expands, you will get an error like "cannot pass value of type struct B to struct A" instead of it either succeeding or giving you some other, unhelpful error. */
SOME_MACRO_TYPE_SAFE(&i_am_a_b_struct);

/*
TODO: explain why (including and beyond the obvious) this may be useful
*/

/* i'm sure this is probably already a very well-known trick among erudites, but i figured i'd put this somewhere for my own reference. if any viewers have comments or notice an oversight, let me know! */