Fix function to pointer conversion

This snippet is valid:
  void foo(void);
  ... foo + 42 ...
the function designator is converted to pointer to function
implicitely.  gen_op didn't do that and bailed out.

tccgen.c

@@ -1926,6 +1926,7 @@ ST_FUNC void gen_op(int op)
     int u, t1, t2, bt1, bt2, t;
     CType type1;
 
+redo:
     t1 = vtop[-1].type.t;
     t2 = vtop[0].type.t;
     bt1 = t1 & VT_BTYPE;
@@ -1933,6 +1934,18 @@ ST_FUNC void gen_op(int op)
         
     if (bt1 == VT_STRUCT || bt2 == VT_STRUCT) {
         tcc_error("operation on a struct");
+    } else if (bt1 == VT_FUNC || bt2 == VT_FUNC) {
+	if (bt2 == VT_FUNC) {
+	    mk_pointer(&vtop->type);
+	    gaddrof();
+	}
+	if (bt1 == VT_FUNC) {
+	    vswap();
+	    mk_pointer(&vtop->type);
+	    gaddrof();
+	    vswap();
+	}
+	goto redo;
     } else if (bt1 == VT_PTR || bt2 == VT_PTR) {
         /* at least one operand is a pointer */
         /* relationnal op: must be both pointers */

tests/tcctest.c

@@ -1847,6 +1847,7 @@ void funcptr_test()
         int dummy;
         void (*func)(int);
     } st1;
+    long diff;
 
     printf("funcptr:\n");
     func = #
@@ -1862,6 +1863,12 @@ void funcptr_test()
     printf("sizeof2 = %d\n", sizeof funcptr_test);
     printf("sizeof3 = %d\n", sizeof(&funcptr_test));
     printf("sizeof4 = %d\n", sizeof &funcptr_test);
+    a = 0;
+    func = num + a;
+    diff = func - num;
+    func(42);
+    (func + diff)(42);
+    (num + a)(43);
 }
 
 void lloptest(long long a, long long b)