CWG2875 Missing support for round-tripping null pointer values through indirection/address operators

Author: eisenwave

source/compatibility.tex

@@ -3041,6 +3041,32 @@
 (for instance, via the C typedef in \libheaderref{stdbool.h})
 is ill-formed in \Cpp{}.
 
+\diffref{expr.unary.op}
+\change
+In certain contexts,
+taking the address of a dereferenced null or past-the-end pointer value
+is well-defined in C (and yields the original pointer value),
+but results in undefined behavior in \Cpp{}.
+For example:
+\begin{codeblock}
+void f() {
+  char *p = nullptr;
+  char *p2 = &*p;       // well-defined in C, undefined behavior in \Cpp{}
+  char *p3 = &p[0];     // well-defined in C, undefined behavior in \Cpp{}
+  int a[5];
+  int *q = &a[5];       // well-defined in C, undefined behavior in \Cpp{}
+}
+\end{codeblock}
+\rationale
+Consistent treatment of lvalues in \Cpp{}.
+\effect
+Well-formed and well-defined C code exhibits undefined behavior in \Cpp{}.
+\difficulty
+Syntactic transformation to pointer arithmetic
+and possible addition of a check for null pointer values.
+\howwide
+Occasionally.
+
 \diffref{expr.sizeof,expr.cast}
 \change
 In \Cpp{}, types can only be defined in declarations, not in expressions.\\