[expr.add] Clarify if/otherwise ladder

source/expressions.tex

@@ -5438,9 +5438,13 @@
 
 \pnum
 \indextext{arithmetic!pointer}%
-When an expression that has integral type is added to or subtracted from
-a pointer, the result has the type of the pointer operand.
-If the expression \tcode{P} points to element $\mathtt{x[}i\mathtt{]}$
+When an expression \tcode{J} that has integral type
+is added to or subtracted from an expression \tcode{P} of pointer type,
+the result has the type of \tcode{P}.
+\begin{itemize}
+\item If \tcode{P} evaluates to a null pointer value and
+\tcode{J} evaluates to 0, the result is a null pointer value.
+\item Otherwise, if \tcode{P} points to element $\mathtt{x[}i\mathtt{]}$
 of an array object \tcode{x} with $n$ elements,%
 \footnote{An object that is not an array element is considered to belong to a
 single-element array for this purpose; see~\ref{expr.unary.op}.
@@ -5450,39 +5454,39 @@
 the expressions \tcode{P + J} and \tcode{J + P}
 (where \tcode{J} has the value $j$)
 point to the (possibly-hypothetical) element
-$\mathtt{x[}i + j\mathtt{]}$ if $0 \le i + j \le n$;
-otherwise, the behavior is undefined.
-Likewise, the expression \tcode{P - J}
+$\mathtt{x[}i + j\mathtt{]}$ if $0 \le i + j \le n$
+and the expression \tcode{P - J}
 points to the (possibly-hypothetical) element
-$\mathtt{x[}i - j\mathtt{]}$ if $0 \le i - j \le n$;
-otherwise, the behavior is undefined.
+$\mathtt{x[}i - j\mathtt{]}$ if $0 \le i - j \le n$.
+\item Otherwise, the behavior is undefined.
+\end{itemize}
 
 \pnum
 \indextext{\idxcode{ptrdiff_t}!implementation-defined type of}%
 \indextext{subtraction!implementation-defined pointer}%
 \indextext{\idxcode{ptrdiff_t}}%
 \indexhdr{cstddef}%
 \indextext{comparison!undefined pointer}%
-When two pointers to elements of the same array object are subtracted,
+When two pointer expressions \tcode{P} and \tcode{Q} are subtracted,
 the type of the result is an \impldef{type of \tcode{ptrdiff_t}} signed
 integral type; this type shall be the same type that is defined as
 \tcode{std::ptrdiff_t} in the \tcode{<cstddef>}
 header\iref{support.types}.
-If the expressions \tcode{P} and \tcode{Q}
-point to, respectively,
-elements
-$\mathtt{x[}i\mathtt{]}$
-and
-$\mathtt{x[}j\mathtt{]}$
+\begin{itemize}
+\item If \tcode{P} and \tcode{Q} both evaluate to null pointer values,
+the result is 0.
+\item Otherwise, if \tcode{P} and \tcode{Q} point to, respectively,
+elements $\mathtt{x[}i\mathtt{]}$ and $\mathtt{x[}j\mathtt{]}$
 of the same array object \tcode{x},
-the expression \tcode{P - Q} has the value $i - j$;
-otherwise, the behavior is undefined.
+the expression \tcode{P - Q} has the value $i - j$.
+\item Otherwise, the behavior is undefined.
 \begin{note}
 If the value $i - j$
 is not in the range of representable values
 of type \tcode{std::ptrdiff_t},
 the behavior is undefined.
 \end{note}
+\end{itemize}
 
 \pnum
 For addition or subtraction, if the expressions \tcode{P} or \tcode{Q} have
@@ -5492,13 +5496,6 @@
 pointer arithmetic when the array contains objects of a derived class type.
 \end{note}
 
-\pnum
-If the value 0 is added to or subtracted from a null pointer value,
-the result is a null pointer value.
-If two null pointer values are subtracted,
-the result compares equal to the value 0
-converted to the type \tcode{std::ptrdiff_t}.
-
 \rSec2[expr.shift]{Shift operators}
 
 \pnum