Merge 2018-06 LWG Motion 14

Fixes #2132

Author: zygoloid

source/lib-intro.tex

@@ -495,37 +495,92 @@
 
 \pnum
 Descriptions of function semantics contain the following elements (as
-appropriate):\footnote{To save space, items that do not apply to a function are omitted.
-For example, if a function does not specify any
-further
-preconditions, there will be no \requires paragraph.}
+appropriate):\footnote{To save space, elements that do not apply to a function are omitted.
+For example, if a function specifies no
+preconditions, there will be no \expects element.}
 
 \begin{itemize}
-\item \requires the preconditions for calling the function
-\item \effects the actions performed by the function
-\item \sync the synchronization operations\iref{intro.multithread} applicable to the function
-\item \postconditions the observable results established by the function
-\item \returns a description of the value(s) returned by the function
-\item \throws any exceptions thrown by the function, and the conditions that would cause the exception
-\item \complexity the time and/or space complexity of the function
-\item \remarks additional semantic constraints on the function
-\item \errors the error conditions for error codes reported by the function
+\item
+\requires the preconditions for calling the function.
+
+\item
+\constraints the conditions for the function's participation
+in overload resolution\iref{over.match}.
+\begin{note}
+Failure to meet such a condition results in the function's silent non-viability.
+\end{note}
+\begin{example}
+An implementation might express such a condition
+via a \grammarterm{constraint-expression}\iref{temp.constr.decl}.
+\end{example}
+
+\item
+\mandates the conditions that, if not met, render the program ill-formed.
+\begin{example}
+An implementation might express such a condition
+via the \grammarterm{constant-expression}
+in a \grammarterm{static\_assert-declaration}\iref{dcl.dcl}.
+If the diagnostic is to be emitted only after the function
+has been selected by overload resolution,
+an implementation might express such a condition
+via a \grammarterm{constraint-expression}\iref{temp.constr.decl}
+and also define the function as deleted.
+\end{example}
+
+\item
+\expects the conditions (sometimes termed preconditions)
+that the function assumes to hold whenever it is called.
+\begin{example}
+An implementation might express such conditions
+via an attribute such as \tcode{[[expects]]}\iref{dcl.attr.contract}.
+However, some such conditions might not lend themselves
+to expression via code.
+\end{example}
+
+\item
+\effects the actions performed by the function.
+
+\item
+\sync the synchronization operations\iref{intro.multithread} applicable to the function.
+
+\item
+\ensures the conditions (sometimes termed observable results or postconditions)
+established by the function.
+
+\item
+\returns a description of the value(s) returned by the function.
+
+\item
+\throws any exceptions thrown by the function, and the conditions that would cause the exception.
+
+\item
+\complexity the time and/or space complexity of the function.
+
+\item
+\remarks additional semantic constraints on the function.
+
+\item
+\errors the error conditions for error codes reported by the function.
 \end{itemize}
 
 \pnum
 Whenever the \Fundescx{Effects} element specifies that the semantics of some function
 \tcode{F} are \term{Equivalent to} some code sequence, then the various elements are
-interpreted as follows. If \tcode{F}'s semantics specifies a \Fundescx{Requires} element, then
-that requirement is logically imposed prior to the \term{equivalent-to} semantics.
-Next, the semantics of the code sequence are determined by the \Fundescx{Requires}, \Fundescx{Effects},
-\Fundescx{Synchronization}, \Fundescx{Postconditions}, \Fundescx{Returns}, \Fundescx{Throws}, \Fundescx{Complexity}, \Fundescx{Remarks}, and \Fundescx{Error conditions}
-specified for the function invocations contained in the code sequence. The value
-returned from \tcode{F} is specified by \tcode{F}'s \Fundescx{Returns} element, or if \tcode{F}
-has no \Fundescx{Returns} element, a non-\tcode{void} return from \tcode{F} is specified by the
+interpreted as follows.
+If \tcode{F}'s semantics specifies any \Fundescx{Constraints} or \Fundescx{Mandates} elements,
+then those requirements are logically imposed prior to the \term{equivalent-to} semantics.
+Next, the semantics of the code sequence are determined by the
+\Fundescx{Constraints}, \Fundescx{Mandates}, \Fundescx{Expects}, \Fundescx{Effects},
+\Fundescx{Synchronization}, \Fundescx{Ensures}, \Fundescx{Returns}, \Fundescx{Throws},
+\Fundescx{Complexity}, \Fundescx{Remarks}, and \Fundescx{Error conditions}
+specified for the function invocations contained in the code sequence.
+The value returned from \tcode{F} is specified by \tcode{F}'s \Fundescx{Returns} element,
+or if \tcode{F} has no \Fundescx{Returns} element,
+a non-\tcode{void} return from \tcode{F} is specified by the
 \tcode{return} statements\iref{stmt.return} in the code sequence.
 If \tcode{F}'s semantics contains a \Fundescx{Throws},
-\Fundescx{Postconditions}, or \Fundescx{Complexity} element, then that supersedes any occurrences of that
-element in the code sequence.
+\Fundescx{Ensures}, or \Fundescx{Complexity} element,
+then that supersedes any occurrences of that element in the code sequence.
 
 \pnum
 For non-reserved replacement and handler functions,
@@ -2737,11 +2792,16 @@
 
 \pnum
 \indextext{restriction}%
-Violation of the preconditions specified in a function's
+Violation of any preconditions specified in a function's
 \requires
-paragraph results in undefined behavior unless the function's
+element results in undefined behavior unless the function's
 \throws
-paragraph specifies throwing an exception when the precondition is violated.
+element specifies throwing an exception when the precondition is violated.
+
+\pnum
+Violation of any preconditions specified
+in a function's \expects element
+results in undefined behavior.
 
 \rSec2[conforming]{Conforming implementations}
 

source/macros.tex

@@ -234,8 +234,12 @@
 \newcommand{\Fundesc}[1]{\Fundescx{#1:}\space}
 \newcommand{\required}{\Fundesc{Required behavior}}
 \newcommand{\requires}{\Fundesc{Requires}}
+\newcommand{\constraints}{\Fundesc{Constraints}}
+\newcommand{\mandates}{\Fundesc{Mandates}}
+\newcommand{\expects}{\Fundesc{Expects}}
 \newcommand{\effects}{\Fundesc{Effects}}
-\newcommand{\postconditions}{\Fundesc{Postconditions}}
+\newcommand{\ensures}{\Fundesc{Ensures}}
+\newcommand{\postconditions}{\ensures}
 \newcommand{\returns}{\Fundesc{Returns}}
 \newcommand{\throws}{\Fundesc{Throws}}
 \newcommand{\default}{\Fundesc{Default behavior}}