Merge 2024-03 CWG Motion 7

P2795R5 Erroneous behaviour for uninitialized reads

Author: tkoeppe

source/basic.tex

@@ -3744,26 +3744,49 @@
 \end{note}
 \indextext{object lifetime|)}
 
-\rSec2[basic.indet]{Indeterminate values}
+\rSec2[basic.indet]{Indeterminate and erroneous values}
 
 \pnum
 \indextext{value!indeterminate}%
 \indextext{indeterminate value}%
 When storage for an object with automatic or dynamic storage duration
-is obtained, the object has an \defnadj{indeterminate}{value}, and if
-no initialization is performed for the object, that object retains an
-indeterminate value until that value is replaced\iref{expr.ass}.
+is obtained,
+the bytes comprising the storage for the object
+have the following initial value:
+\begin{itemize}
+\item
+If the object has dynamic storage duration, or
+is the object associated with a variable or function parameter
+whose first declaration is marked with
+the \tcode{[[indeterminate]]} attribute\iref{dcl.attr.indet},
+the bytes have \defnadjx{indeterminate}{values}{value};
+\item
+otherwise, the bytes have erroneous values,
+where each value is determined by the implementation
+independently of the state of the program.
+\end{itemize}
+If no initialization is performed for an object (including subobjects),
+such a byte retains its initial value
+until that value is replaced\iref{dcl.init.general,expr.ass}.
+If any bit in the value representation has an indeterminate value,
+the object has an indeterminate value;
+otherwise, if any bit in the value representation has an erroneous value,
+the object has an erroneous value\iref{conv.lval}.
 \begin{note}
 Objects with static or thread storage duration are zero-initialized,
 see~\ref{basic.start.static}.
 \end{note}
 
 \pnum
-If an indeterminate value is produced by an evaluation,
-the behavior is undefined except in the following cases:
+Except in the following cases,
+if an indeterminate value is produced by an evaluation,
+the behavior is undefined, and
+if an erroneous value is produced by an evaluation,
+the behavior is erroneous and
+the result of the evaluation is the value so produced but is not erroneous:
 \begin{itemize}
 \item
-  If an indeterminate value of
+  If an indeterminate or erroneous value of
   unsigned ordinary character type\iref{basic.fundamental}
   or \tcode{std::byte} type\iref{cstddef.syn}
   is produced by the evaluation of:
@@ -3780,37 +3803,69 @@
   \item
     a discarded-value expression\iref{expr.context},
   \end{itemize}
-  then the result of the operation is an indeterminate value.
+  then the result of the operation is an indeterminate value or
+  that errorneous value, respectively.
 \item
-  If an indeterminate value of
+  If an indeterminate or erroneous value of
   unsigned ordinary character type or \tcode{std::byte} type
   is produced by the evaluation of
   the right operand of a simple assignment operator\iref{expr.ass}
   whose first operand is an lvalue of
   unsigned ordinary character type or \tcode{std::byte} type,
-  an indeterminate value replaces
+  an indeterminate value or that erroneous value, respectively, replaces
   the value of the object referred to by the left operand.
 \item
-  If an indeterminate value of unsigned ordinary character type
+  If an indeterminate or erroneous value of unsigned ordinary character type
   is produced by the evaluation of the initialization expression
   when initializing an object of unsigned ordinary character type,
   that object is initialized to an indeterminate
-  value.
+  value or that erroneous value, respectively.
 \item
   If an indeterminate value of
   unsigned ordinary character type or \tcode{std::byte} type
   is produced by the evaluation of the initialization expression
   when initializing an object of \tcode{std::byte} type,
-  that object is initialized to an indeterminate value.
+  that object is initialized to an indeterminate value or
+  that erroneous value, respectively.
 \end{itemize}
+Converting an indeterminate or erroneous value of
+unsigned ordinary character type or \tcode{std::byte} type
+produces an indeterminate or erroneous value, respectively.
+In the latter case,
+the result of the conversion is the value of the converted operand.
 \begin{example}
 \begin{codeblock}
 int f(bool b) {
-  unsigned char c;
-  unsigned char d = c;          // OK, \tcode{d} has an indeterminate value
+  unsigned char *c = new unsigned char;
+  unsigned char d = *c;         // OK, \tcode{d} has an indeterminate value
   int e = d;                    // undefined behavior
   return b ? d : 0;             // undefined behavior if \tcode{b} is \tcode{true}
 }
+
+int g(bool b) {
+  unsigned char c;
+  unsigned char d = c;          // no erroneous behavior, but \tcode{d} has an erroneous value
+
+  assert(c == d);               // holds, both integral promotions have erroneous behavior
+
+  int e = d;                    // erroneous behavior
+  return b ? d : 0;             // erroneous behavior if \tcode{b} is \tcode{true}
+}
+
+void h() {
+  int d1, d2;
+
+  int e1 = d1;                  // erroneous behavior
+  int e2 = d1;                  // erroneous behavior
+
+  assert(e1 == e2);             // holds
+  assert(e1 == d1);             // holds, erroneous behavior
+  assert(e2 == d1);             // holds, erroneous behavior
+
+  std::memcpy(&d2, &d1, sizeof(int));   // no erroneous behavior, but \tcode{d2} has an erroneous value
+  assert(e1 == d2);             // holds, erroneous behavior
+  assert(e2 == d2);             // holds, erroneous behavior
+}
 \end{codeblock}
 \end{example}
 

source/classes.tex

@@ -5505,7 +5505,7 @@
 is neither initialized nor
 given a value
 during execution of the \grammarterm{compound-statement} of the body of the constructor,
-the member has an indeterminate value.
+the member has an indeterminate or erroneous value\iref{basic.indet}.
 \end{note}
 \begin{example}
 \begin{codeblock}
@@ -5521,7 +5521,7 @@
   C() { }               // initializes members as follows:
   A a;                  // OK, calls \tcode{A::A()}
   const B b;            // error: \tcode{B} has no default constructor
-  int i;                // OK, \tcode{i} has indeterminate value
+  int i;                // OK, \tcode{i} has indeterminate or erroneous value
   int j = 5;            // OK, \tcode{j} has the value \tcode{5}
 };
 \end{codeblock}

source/compatibility.tex

@@ -2881,8 +2881,8 @@
 \end{codeblock}
 
 \rationale
-This is to avoid erroneous function calls (i.e., function calls
-with the wrong number or type of arguments).
+This is to avoid function calls
+with the wrong number or type of arguments.
 \effect
 Change to semantics of well-defined feature.
 This feature was marked as ``obsolescent'' in C.

source/declarations.tex

@@ -9189,6 +9189,52 @@
 \end{codeblock}
 \end{example}
 
+\rSec2[dcl.attr.indet]{Indeterminate storage}
+\indextext{attribute!indeterminate}
+
+\pnum
+The \grammarterm{attribute-token} \tcode{indeterminate} may be applied
+to the definition of a block variable with automatic storage duration or
+to a \grammarterm{parameter-declaration} of a function declaration.
+No \grammarterm{attribute-argument-clause} shall be present.
+The attribute specifies
+that the storage of an object with automatic storage duration
+is initially indeterminate rather than erroneous\iref{basic.indet}.
+
+\pnum
+If a function parameter is declared with the \tcode{indeterminate} attribute,
+it shall be so declared in the first declaration of its function.
+If a function parameter is declared with
+the \tcode{indeterminate} attribute in the first declaration of its function
+in one translation unit and
+the same function is declared without the \tcode{indeterminate} attribute
+on the same parameter in its first declaration in another translation unit,
+the program is ill-formed, no diagnostic required.
+
+\pnum
+\begin{note}
+Reading from an uninitialized variable
+that is marked \tcode{[[indeterminate]]} can cause undefined behavior.
+\begin{codeblock}
+void f(int);
+void g() {
+  int x [[indeterminate]], y;
+  f(y);                 // erroneous behavior\iref{basic.indet}
+  f(x);                 // undefined behavior
+}
+
+struct T {
+  T() {}
+  int x;
+};
+int h(T t [[indeterminate]]) {
+  f(t.x);               // undefined behavior when called below
+  return 0;
+}
+int _ = h(T());
+\end{codeblock}
+\end{note}
+
 \rSec2[dcl.attr.likelihood]{Likelihood attributes}%
 \indextext{attribute!likely}
 \indextext{attribute!unlikely}

source/expressions.tex

@@ -675,6 +675,9 @@
 
 \item Otherwise, the object indicated by the glvalue is read\iref{defns.access},
 and the value contained in the object is the prvalue result.
+If the result is an erroneous value\iref{basic.indet} and
+the bits in the value representation are not valid for the object's type,
+the behavior is undefined.
 \end{itemize}
 
 \pnum
@@ -5299,7 +5302,7 @@
 If the \grammarterm{expression} in a \grammarterm{noptr-new-declarator}
 is present, it is implicitly converted to \tcode{std::size_t}.
 \indextext{function!allocation}%
-The \grammarterm{expression} is erroneous if:
+The value of the \grammarterm{expression} is invalid if:
 \begin{itemize}
 \item
 the expression is of non-class type and its value before converting to
@@ -5327,7 +5330,7 @@
 number of elements to initialize.
 \end{itemize}
 
-If the \grammarterm{expression} is erroneous after converting to \tcode{std::size_t}:
+If the value of the \grammarterm{expression} is invalid after converting to \tcode{std::size_t}:
 \begin{itemize}
 \item
 if the \grammarterm{expression} is a potentially-evaluated core constant expression,
@@ -7519,7 +7522,7 @@
 limits (see \ref{implimits});
 
 \item
-an operation that would have undefined behavior
+an operation that would have undefined or erroneous behavior
 as specified in \ref{intro} through \ref{cpp},
 excluding \ref{dcl.attr.assume} and \ref{dcl.attr.noreturn};
 \begin{footnote}
@@ -7937,7 +7940,7 @@
 
   \item
   if the value is an object of scalar type,
-  it does not have an indeterminate value\iref{basic.indet},
+  it does not have an indeterminate or erroneous value\iref{basic.indet},
 
   \item
   if the value is of pointer type, it contains
@@ -7973,6 +7976,14 @@
 constexpr int r = h();                          // OK
 constexpr auto e = g();                         // error: a pointer to an immediate function is
                                                 // not a permitted result of a constant expression
+
+struct S {
+  int x;
+  constexpr S() {}
+};
+int i() {
+  constexpr S s;                                // error: \tcode{s.x} has erroneous value
+}
 \end{codeblock}
 \end{example}
 

source/intro.tex

@@ -253,6 +253,16 @@
 \definition{dynamic type}{defns.dynamic.type.prvalue}
 \defncontext{prvalue} \termref{defns.static.type}{static type}{} of the prvalue expression
 
+\definition{erroneous behavior}{defns.erroneous}
+well-defined behavior that the implementation is recommended to diagnose
+\begin{defnote}
+Erroneous behavior is always the consequence of incorrect program code.
+Implementations are allowed, but not required,
+to diagnose it\iref{intro.compliance.general}.
+Evaluation of a constant expression\iref{expr.const}
+never exhibits behavior specified as erroneous in \ref{intro} through \ref{cpp}.
+\end{defnote}
+
 \definition{expression-equivalent}{defns.expression.equivalent}
 \defncontext{library}
 \indexdefn{expression-equivalent}%
@@ -629,13 +639,13 @@
 \begin{defnote}
 Undefined behavior may be expected when
 this document omits any explicit
-definition of behavior or when a program uses an erroneous construct or erroneous data.
+definition of behavior or when a program uses an incorrect construct or invalid data.
 Permissible undefined behavior ranges
 from ignoring the situation completely with unpredictable results, to
 behaving during translation or program execution in a documented manner
 characteristic of the environment (with or without the issuance of a
 \termref{defns.diagnostic}{diagnostic message}{}), to terminating a translation or execution (with the
-issuance of a diagnostic message). Many erroneous program constructs do
+issuance of a diagnostic message). Many incorrect program constructs do
 not engender undefined behavior; they are required to be diagnosed.
 Evaluation of a constant expression\iref{expr.const} never exhibits behavior explicitly
 specified as undefined in \ref{intro} through \ref{cpp}.
@@ -721,7 +731,8 @@
 within its resource limits as described in \ref{implimits},
 accept and correctly execute
 \begin{footnote}
-``Correct execution'' can include undefined behavior, depending on
+``Correct execution'' can include undefined behavior
+and erroneous behavior, depending on
 the data being processed; see \ref{intro.defs} and~\ref{intro.execution}.
 \end{footnote}
 that program.
@@ -900,6 +911,20 @@
 requirement on the implementation executing that program with that input
 (not even with regard to operations preceding the first undefined
 operation).
+If the execution contains an operation specified as having erroneous behavior,
+the implementation is permitted to issue a diagnostic and
+is permitted to terminate the execution
+at an unspecified time after that operation.
+
+\pnum
+\recommended
+An implementation should issue a diagnostic when such an operation is executed.
+\begin{note}
+An implementation can issue a diagnostic
+if it can determine that erroneous behavior is reachable
+under an implementation-specific set of assumptions about the program behavior,
+which can result in false positives.
+\end{note}
 
 \pnum
 \indextext{conformance requirements}%

source/lib-intro.tex

@@ -1939,10 +1939,10 @@
 \pnum
 A value-initialized object of type \tcode{P} produces the null value of the type.
 The null value shall be equivalent only to itself. A default-initialized object
-of type \tcode{P} may have an indeterminate value.
+of type \tcode{P} may have an indeterminate or erroneous value.
 \begin{note}
-Operations involving
-indeterminate values can cause undefined behavior.
+Operations involving indeterminate values can cause undefined behavior, and
+operations involving erroneous values can cause erroneous behavior\iref{basic.indet}.
 \end{note}
 
 \pnum

source/threads.tex

@@ -8376,7 +8376,7 @@
 \pnum
 \begin{note}
 It is the user's responsibility to ensure that waiting threads
-do not erroneously assume that the thread has finished if they experience
+do not incorrectly assume that the thread has finished if they experience
 spurious wakeups. This typically requires that the condition being waited
 for is satisfied while holding the lock on \tcode{lk}, and that this lock
 is not released and reacquired prior to calling \tcode{notify_all_at_thread_exit}.

source/utilities.tex

@@ -18865,13 +18865,27 @@
 If there are multiple such values, which value is produced is unspecified.
 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 of an object that is not within its lifetime or
+corresponds to a bit
+for which the smallest enclosing object is not within its lifetime or
 has an indeterminate value\iref{basic.indet}.
-For each bit in the value representation of the result that is indeterminate,
-the smallest object containing that bit has an indeterminate value;
-the behavior is undefined unless that object is
-of unsigned ordinary character type or \tcode{std::byte} type.
-The result does not otherwise contain any indeterminate values.
+A bit in the value representation of the result is erroneous
+if it corresponds to a bit
+for which the smallest enclosing object has an erroneous value.
+For each bit $b$ in the value representation of the result
+that is indeterminate or erroneous,
+let $u$ be the smallest object containing that bit enclosing $b$:
+\begin{itemize}
+\item
+If $u$ is of unsigned ordinary character type or \tcode{std::byte} type,
+$u$ has an indeterminate value
+if any of the bits in its value representation are indeterminate, or
+otherwise has an erroneous value.
+\item
+Otherwise, if $b$ is indeterminate, the behavior is undefined.
+\item
+Otherwise, the behaviour is erroneous, and the result is as specified above.
+\end{itemize}
+The result does not otherwise contain any indeterminate or erroneous values.
 
 \pnum
 \remarks