To reproduce:

1) Compile the following Vala program:

   //----------------
   void glitch(void *vp) {
   	int i = (int)(*vp);
   	stderr.printf("\ni = %d.\n", i);
   }

   int main()
   {
   	int i = 4;
   	glitch(&i);
   	return 0;
   }
   //----------------

and observe the error returned from the C compiler.

Observed: valac accepts this program, generating invalid C code (error: invalid use of void expression).

Expected: valac displays an error message informing the user that trying to extract a value from a void expression is forbidden.