Merge 2019-07 CWG Motion 7

 P1301R4 [[nodiscard("should have a reason")]]

Fixes #2986.

Author: zygoloid

source/declarations.tex

@@ -9025,9 +9025,9 @@
 unsafe. \end{note} It shall appear at most once in each \grammarterm{attribute-list}. An
 \grammarterm{attribute-argument-clause} may be present and, if present, it shall have the form:
 
-\begin{codeblock}
-( @\grammarterm{string-literal}@ )
-\end{codeblock}
+\begin{ncbnf}
+\terminal{(} string-literal \terminal{)}
+\end{ncbnf}
 \begin{note} The \grammarterm{string-literal} in the \grammarterm{attribute-argument-clause}
 could be used to explain the rationale for deprecation and/or to suggest a replacing entity.
 \end{note}
@@ -9235,8 +9235,29 @@
 The \grammarterm{attribute-token} \tcode{nodiscard}
 may be applied to the \grammarterm{declarator-id}
 in a function declaration or to the declaration of a class or enumeration.
-It shall appear at most once in each \grammarterm{attribute-list} and
-no \grammarterm{attribute-argument-clause} shall be present.
+It shall appear at most once in each \grammarterm{attribute-list}.
+An \grammarterm{attribute-argument-clause} may be present
+and, if present, it shall have the form:
+
+\begin{ncbnf}
+\terminal{(} string-literal \terminal{)}
+\end{ncbnf}
+
+\pnum
+A name or entity declared without the \tcode{nodiscard} attribute
+can later be redeclared with the attribute and vice-versa.
+\begin{note}
+Thus, an entity initially declared without the attribute
+can be marked as \tcode{nodiscard}
+by a subsequent redeclaration.
+However, after an entity is marked as \tcode{nodiscard},
+later redeclarations do not remove the \tcode{nodiscard}
+from the entity.
+\end{note}
+Redeclarations using different forms of the attribute
+(with or without the \grammarterm{attribute-argument-clause}
+or with different \grammarterm{attribute-argument-clause}s)
+are allowed.
 
 \pnum
 \begin{note}
@@ -9249,6 +9270,10 @@
 Implementations should issue a warning in such cases.
 This is typically because discarding the return value
 of a nodiscard call has surprising consequences.
+The \grammarterm{string-literal}
+in a \tcode{nodiscard} \grammarterm{attribute-argument-clause}
+is intended to be used in the message of the warning
+as the rationale for why the result should not be discarded.
 \end{note}
 
 \pnum

source/preprocessor.tex

@@ -308,7 +308,7 @@
 \tcode{likely}                & \tcode{201803L} \\
 \tcode{maybe_unused}          & \tcode{201603L} \\
 \tcode{no_unique_address}     & \tcode{201803L} \\
-\tcode{nodiscard}             & \tcode{201603L} \\
+\tcode{nodiscard}             & \tcode{201907L} \\
 \tcode{noreturn}              & \tcode{200809L} \\
 \tcode{unlikely}              & \tcode{201803L} \\
 \end{floattable}