[CWG 2026-06-7] P2434R5 Nondeterministic pointer provenance

source/basic.tex

@@ -5166,57 +5166,6 @@
 or functions\iref{dcl.fct}.
 \end{note}
 
-\pnum
-\indextext{object!byte copying and|(}%
-\indextext{type!trivially copyable}%
-For any object (other than a potentially-overlapping subobject) of trivially copyable type
-\tcode{T}, whether or not the object holds a valid value of type
-\tcode{T}, the underlying bytes\iref{intro.memory} making up the
-object can be copied into an array of
-\keyword{char},
-\tcode{\keyword{unsigned} \keyword{char}}, or
-\tcode{std::byte}\iref{cstddef.syn}.
-\begin{footnote}
-By using, for example, the library
-functions\iref{headers} \tcode{std::memcpy} or \tcode{std::memmove}.
-\end{footnote}
-If the content of that array
-is copied back into the object, the object shall
-subsequently hold its original value.
-\begin{example}
-\begin{codeblock}
-constexpr std::size_t N = sizeof(T);
-char buf[N];
-T obj;                          // \tcode{obj} initialized to its original value
-std::memcpy(buf, &obj, N);      // between these two calls to \tcode{std::memcpy}, \tcode{obj} might be modified
-std::memcpy(&obj, buf, N);      // at this point, each subobject of \tcode{obj} of scalar type holds its original value
-\end{codeblock}
-\end{example}
-
-\pnum
-For two distinct objects \tcode{obj1} and \tcode{obj2}
-of trivially copyable type \tcode{T},
-where neither \tcode{obj1} nor \tcode{obj2} is a potentially-overlapping subobject,
-if the underlying bytes\iref{intro.memory} making up
-\tcode{obj1} are copied into \tcode{obj2},
-\begin{footnote}
-By using, for example,
-the library functions\iref{headers} \tcode{std::memcpy} or \tcode{std::memmove}.
-\end{footnote}
- \tcode{obj2} shall subsequently hold the same value as
-\tcode{obj1}.
-\begin{example}
-\begin{codeblock}
-T* t1p;
-T* t2p;
-    // provided that \tcode{t2p} points to an initialized object ...
-std::memcpy(t1p, t2p, sizeof(T));
-    // at this point, every subobject of trivially copyable type in \tcode{*t1p} contains
-    // the same value as the corresponding subobject in \tcode{*t2p}
-\end{codeblock}
-\end{example}
-\indextext{object!byte copying and|)}
-
 \pnum
 \label{term.object.representation}%
 The \defnx{object representation}{representation!object}
@@ -5243,10 +5192,6 @@
 Bits in the object representation of a type or object that are
 not part of the value representation
 are \defn{padding bits}.
-For trivially copyable types, the value representation is
-a set of bits in the object representation that determines a
-\defn{value}, which is one discrete element of an
-\impldef{values of a trivially copyable type} set of values.
 \begin{footnote}
 The intent is that the memory model of \Cpp{} is compatible
 with that of the C programming language.
@@ -5424,6 +5369,88 @@
 Every function of consteval-only type shall be
 an immediate function\iref{expr.const.imm}.
 
+\rSec2[basic.types.trivial]{Trivially copyable types}
+
+%%FIXME: This paragraph is hard to understand.
+\pnum
+Each trivially copyable type \tcode{T} has an
+\impldef{values of a trivially copyable type}
+set of discrete \defnx{values}{value}.
+\indextext{value!representation}%
+Each possible value representation of an object of type \tcode{T}
+corresponds to a distinct
+\impldef{subset of values of a trivially copyable type
+         for which a value representation corresponds to}
+subset of this set.
+%%FIXME: "*the* set of values" - what set of values?
+The union of these subsets is the set of values;
+for scalar types other than object pointer types,
+each such subset contains no more than one value.
+Certain operations cause an object to
+\indextext{object!acquiring a value representation}%
+\indextext{value!representation!aquire}%
+\defn{acquire} a value representation,
+in which case the object's value
+is replaced with an unspecified member of the corresponding subset that
+would result in the program having defined behavior, if any.
+
+\pnum
+\begin{note}
+A single subset for a pointer type
+can contain pointers to multiple objects
+in each of several regions of storage whose durations are disjoint.
+\end{note}
+
+\pnum
+\indextext{object!byte copying and|(}%
+\indextext{type!trivially copyable}%
+If an object of such a type \tcode{T}
+is not a potentially-overlapping subobject,
+whether or not the object holds a valid value of type
+\tcode{T}, the underlying bytes\iref{intro.memory} making up the
+object can be copied into an array of
+\keyword{char},
+\tcode{\keyword{unsigned} \keyword{char}}, or
+\tcode{std::byte}\iref{cstddef.syn}.
+\begin{footnote}
+By using, for example, the library
+functions\iref{headers} \tcode{std::memcpy} or \tcode{std::memmove}.
+\end{footnote}
+If the content of that array
+is copied back into the object,
+the object acquires its original value representation.
+\begin{example}
+\begin{codeblock}
+constexpr std::size_t N = sizeof(T);
+char buf[N];
+T obj;                          // \tcode{obj} initialized to its original value
+std::memcpy(buf, &obj, N);      // between these two calls to \tcode{std::memcpy}, \tcode{obj} might be modified
+std::memcpy(&obj, buf, N);      // at this point, each subobject of \tcode{obj} of scalar type holds its original value
+\end{codeblock}
+\end{example}
+
+\pnum
+For two distinct such objects \tcode{obj1} and \tcode{obj2},
+if the underlying bytes\iref{intro.memory} making up
+\tcode{obj1} are copied into \tcode{obj2},
+\begin{footnote}
+By using, for example,
+the library functions\iref{headers} \tcode{std::memcpy} or \tcode{std::memmove}.
+\end{footnote}
+ \tcode{obj2} acquires the value representation of
+\tcode{obj1}.
+\begin{example}
+\begin{codeblock}
+T* t1p;
+T* t2p;
+    // provided that \tcode{t2p} points to an initialized object ...
+std::memcpy(t1p, t2p, sizeof(T));
+    // at this point, every subobject of trivially copyable type in \tcode{*t1p} contains
+    // the same value as the corresponding subobject in \tcode{*t2p}
+\end{codeblock}
+\end{example}
+\indextext{object!byte copying and|)}
+
 \rSec2[basic.fundamental]{Fundamental types}
 
 \pnum
@@ -6076,13 +6103,26 @@
 is also associated with $E$.
 \end{note}
 
+\pnum
+If an evaluation produces or causes an object
+to have\iref{basic.types.trivial} a pointer value
+to or past the end of an object $O$ and
+happens before the beginning of the duration
+of the region of storage for $O$,
+the behavior is undefined.
+\begin{note}
+Relaxed atomic operations\iref{atomics.order} can produce such values.
+Conversions from integers avoid producing them\iref{expr.reinterpret.cast}.
+\end{note}
+
 \pnum
 A pointer value $P$ is
 \indextext{value!valid in the context of an evaluation}%
 \defn{valid in the context of} an evaluation $E$
 if $P$ is a pointer to function or a null pointer value, or
 if it is a pointer to or past the end of an object $O$ and
-$E$ happens before the end of the duration of the region of storage for $O$.
+$E$ happens after the beginning
+and happens before the end of the duration of the region of storage for $O$.
 If a pointer value $P$ is used in an evaluation $E$ and
 $P$ is not valid in the context of $E$,
 then the behavior is undefined if $E$ is

source/expressions.tex

@@ -4955,7 +4955,7 @@
 \indextext{cast!reinterpret!pointer to integer}%
 \indextext{cast!pointer to integer}%
 A pointer can be explicitly converted to any integral type large enough
-to hold all values of its type.
+to distinguish all value representations of its type.
 \indextext{conversion!implementation-defined pointer integer}%
 The mapping function is \impldef{mapping of pointer to integer}.
 \begin{note}
@@ -4975,12 +4975,26 @@
 \indextext{cast!reinterpret!integer to pointer}%
 \indextext{cast!integer to pointer}%
 A value of integral type or enumeration type can be explicitly converted
-to a pointer. A pointer converted to an integer of sufficient size (if
-any such exists on the implementation) and back to the same pointer type
-will have its original value\iref{basic.compound};
+to a pointer.
+If the value is one that can be produced
+by converting one or more pointer values\iref{basic.compound}
+to an integral type,
+the result is an unspecified choice among all such values
+that would result in the program having defined behavior.
+If no such value exists, the behavior is undefined.
+\begin{note}
+It is possible for the result
+to be an invalid pointer value or
+to not be valid in the context of the conversion\iref{basic.compound}
+because it points to an object in a region of storage
+whose duration has ended or has not yet begun.
+\end{note}
 \indextext{conversion!implementation-defined pointer integer}%
-mappings between pointers and integers are otherwise
+Otherwise, the result is
 \impldef{conversions between pointers and integers}.
+\begin{note}
+It can be an invalid pointer value.
+\end{note}
 
 \pnum
 \indextext{cast!reinterpret!pointer-to-function}%
@@ -7653,6 +7667,14 @@
 result of type \keyword{bool}. In each case below, the operands shall have the
 same type after the specified conversions have been applied.
 
+\pnum
+Any two pointer values or two pointer-to-member values
+either compare equal or compare unequal.
+\begin{note}
+Repeated comparisons are consistent
+so long as neither value is an invalid pointer value.
+\end{note}
+
 \pnum
 \indextext{comparison!pointer}%
 \indextext{comparison!pointer to function}%
@@ -7772,8 +7794,7 @@
 If two operands compare equal, the result is \keyword{true} for
 the \tcode{==} operator and \keyword{false} for the \tcode{!=} operator. If two operands
 compare unequal, the result is \keyword{false} for the \tcode{==} operator and
-\keyword{true} for the \tcode{!=} operator. Otherwise, the result of each of the
-operators is unspecified.
+\keyword{true} for the \tcode{!=} operator.
 
 \pnum
 If both operands are of arithmetic or enumeration type, the usual arithmetic

source/iostreams.tex

@@ -18997,6 +18997,15 @@
 to be written to a file (\xrefc{7.23.3})
 is an observable checkpoint\iref{intro.abstract}.
 
+\pnum
+When the input item for the \tcode{\%p} conversion
+of the \tcode{fscanf} function (or equivalent)
+is one that can be produced
+from more than one pointer value\iref{basic.compound},
+the pointer that results is an unspecified choice among all those values
+that would result in the program having defined behavior.
+If no such value exists, the behavior is undefined.
+
 \pnum
 Calls to the function \tcode{tmpnam} with an argument that is a null pointer value may
 introduce a data race\iref{res.on.data.races} with other calls to \tcode{tmpnam} with

source/utilities.tex

@@ -16376,10 +16376,11 @@
 Each bit of the value representation of the result
 is equal to the corresponding bit in the object representation
 of \tcode{from}. Padding bits of the result are unspecified.
-For the result and each object created within it,
-if there is no value of the object's type corresponding to the
-value representation produced, the behavior is undefined.
-If there are multiple such values, which value is produced is unspecified.
+Every trivially copyable object
+among the result and each object created within it
+acquires the value representation produced;
+if any such object does not receive a value,
+the behavior is undefined.
 A bit in the value representation of the result is indeterminate if
 it does not correspond to a bit in the value representation of \tcode{from} or
 corresponds to a bit