[zlaski/bad-addition-qhelp-reword] Initial change.

Author: zlaski-semmle

cpp/ql/src/Likely Bugs/Arithmetic/BadAdditionOverflowCheck.qhelp

@@ -6,7 +6,8 @@
     <p>
       Checking for overflow of integer addition needs to be done with
       care, because automatic type promotion can prevent the check
-      from working correctly.
+      from working as intended, with the same value (<code>true</code>
+      or <code>false</code>) always being returned.
     </p>
   </overview>
   <recommendation>
@@ -18,15 +19,16 @@
   <example>
     <sample src="BadAdditionOverflowCheckExample1.cpp" />
     <p>
-      On a typical architecture where <tt>short</tt> is 16 bits
-      and <tt>int</tt> is 32 bits, the operands of the addition are
-      automatically promoted to <tt>int</tt>, so it cannot overflow
+      On a typical architecture where <code>short</code> is 16 bits
+      and <code>int</code> is 32 bits, the operands of the addition are
+      automatically promoted to <code>int</code>, so it cannot overflow
       and the result of the comparison is always false.
     </p>
     <p>
       The code below implements the check correctly, by using an
       explicit cast to make sure that the result of the addition
-      is <tt>unsigned short</tt>.
+      is <code>unsigned short</code> (which may overflow, in which case
+      the comparison would evaluate to <code>true</code>).
     </p>
     <sample src="BadAdditionOverflowCheckExample2.cpp" />
   </example>

cpp/ql/src/Likely Bugs/Arithmetic/BadAdditionOverflowCheckExample1.cpp

@@ -1,3 +1,4 @@
 bool checkOverflow(unsigned short x, unsigned short y) {
-  return (x + y < x);  // BAD: x and y are automatically promoted to int.
+  // BAD: comparison is always false due to type promotion
+  return (x + y < x);  
 }