Copying values to a tmp with __typeof__ "doesn't" work

typeof_fail.c

// Copying variables into a tmp using __typeof__ is a common
// pattern in hacky gcc C macros, but it can backfire and
// allow the value to be evaluated twice!
// This is because __typeof__ will evaluate its argument if
// the value has a variably sized type!
// gcc added __auto_type to get around this problem in its stdatomic.h, but I'm
// not losing any sleep over this.

// See http://patchwork.ozlabs.org/patch/290802/

#include <stdio.h>
int wtf() { printf("wtf\n"); return 0; }
int s = 5;

// Copy a value to the variable tmp
#define TMP_THING(x) __typeof__(x) tmp = (x)

int main(void) {
	int vla[s][s];
	int (*p)[s] = &vla[0];
	// wtf printed twice!
	TMP_THING(p + wtf());
	return 0;
}