CWG2924 Undefined behavior during constant evaluation

Author: burblebee

source/declarations.tex

@@ -9015,6 +9015,7 @@
 
 \rSec2[dcl.attr.assume]{Assumption attribute}
 
+% FIXME: Shouldn't there be a pnum here?
 The \grammarterm{attribute-token} \tcode{assume} may be applied to a null statement;
 such a statement is an \defn{assumption}.
 An \grammarterm{attribute-argument-clause} shall be present and
@@ -9027,7 +9028,8 @@
 If the converted expression would evaluate to \tcode{true}
 at the point where the assumption appears,
 the assumption has no effect.
-Otherwise, the behavior is undefined.
+Otherwise,
+evaluation of the assumption has runtime-undefined behavior.
 \begin{note}
 The expression is potentially evaluated\iref{basic.def.odr}.
 The use of assumptions is intended to allow implementations
@@ -9534,8 +9536,9 @@
 translation unit, the program is ill-formed, no diagnostic required.
 
 \pnum
-If a function \tcode{f} is called where \tcode{f} was previously declared with the \tcode{noreturn}
-attribute and \tcode{f} eventually returns, the behavior is undefined.
+If a function \tcode{f} is invoked where \tcode{f} was previously declared with the \tcode{noreturn}
+attribute and that invocation eventually returns,
+the behavior is runtime-undefined.
 \begin{note}
 The function can
 terminate by throwing an exception.

source/expressions.tex

@@ -7596,8 +7596,7 @@
 
 \item
 an operation that would have undefined or erroneous behavior
-as specified in \ref{intro} through \ref{\lastcorechapter},
-excluding \ref{dcl.attr.assume} and \ref{dcl.attr.noreturn};
+as specified in \ref{intro} through \ref{\lastcorechapter};
 \begin{footnote}
 This includes,
 for example, signed integer overflow\iref{expr.pre}, certain
@@ -7778,33 +7777,23 @@
 \end{note}
 \end{itemize}
 
+\pnum
+It is
+\impldef{whether an expression is a core constant expression}
+whether $E$ is a core constant expression
+if $E$ satisfies the constraints of a core constant expression, but
+evaluation of $E$ has runtime-undefined behavior.
+
 \pnum
 It is unspecified whether $E$ is a core constant expression
 if $E$ satisfies the constraints of a core constant expression, but
 evaluation of $E$ would evaluate
 \begin{itemize}
 \item
 an operation that has undefined behavior
-as specified in \ref{library} through \ref{\lastlibchapter},
+as specified in \ref{library} through \ref{\lastlibchapter} or
 \item
-an invocation of the \tcode{va_start} macro\iref{cstdarg.syn},
-\item
-a call to a function
-that was previously declared
-with the \tcode{noreturn} attribute\iref{dcl.attr.noreturn} and
-that call returns to its caller, or
-\item
-a statement with an assumption\iref{dcl.attr.assume}
-whose converted \grammarterm{conditional-expression},
-if evaluated where the assumption appears,
-would not disqualify $E$ from being a core constant expression and
-would not evaluate to \tcode{true}.
-\begin{note}
-$E$ is not disqualified from being a core constant expression
-if the hypothetical evaluation of
-the converted \grammarterm{conditional-expression}
-would disqualify $E$ from being a core constant expression.
-\end{note}
+an invocation of the \tcode{va_start} macro\iref{cstdarg.syn}.
 \end{itemize}
 
 \pnum

source/intro.tex

@@ -206,6 +206,11 @@
 constructs that it does not support.
 \end{defnote}
 
+\definition{constant evaluation}{defns.const.eval}
+\indexdefn{constant evaluation}%
+evaluation that is performed as part of evaluating an expression
+as a core constant expression\iref{expr.const}
+
 \definition{constant subexpression}{defns.const.subexpr}
 \indexdefn{constant subexpression}%
 expression whose evaluation as subexpression of a
@@ -504,6 +509,23 @@
 \indextext{undefined}
 \end{defnote}
 
+\definition{runtime-undefined behavior}{defns.undefined.runtime}
+\indexdefn{behavior!runtime-undefined}%
+behavior that is undefined except when it occurs during constant evaluation
+
+\begin{defnote}
+During constant evaluation,
+\begin{itemize}
+\item
+it is
+\impldef{whether runtime-undefined behavior results in the expression being deemed non-constant}
+whether runtime-undefined behavior results in the expression being deemed non-constant
+(as specified in~\ref{expr.const}) and
+\item
+runtime-undefined behavior has no other effect.
+\end{itemize}
+\end{defnote}
+
 \indexdefn{signature}%
 \definition{signature}{defns.signature}
 \defncontext{function}