Avoid overflow/underflow; tweak example

Author: ntrel

spec/operatoroverloading.dd

@@ -520,17 +520,22 @@ $(SPEC_RUNNABLE_EXAMPLE_RUN
 struct S
 {
     int i, j;
-    int opCmp(ref const S s) const { return i - s.i; } // ignore j
+    int opCmp(ref const S s) const { return (i > s.i) - (i < s.i); } // ignore j
 }
 
 S a = {2, 3};
 S b = {2, 1};
+S c = {3, 0};
+assert(a < c);
 assert(a <= b);
-assert(!(a < b));
-assert(a != b); // generated opEquals tests both i and j members
+assert(!(a < b)); // opCmp ignores j
+assert(a != b);   // generated opEquals tests both i and j members
 ---
 )
 
+$(BEST_PRACTICE Using `(i > s.i) - (i < s.i)` instead of `i - s.i` to
+compare integers avoids overflow/underflow.)
+
 
 $(H2 $(LEGACY_LNAME2 FunctionCall, function-call, Function Call Operator Overloading $(D f())))