P2741R3 User-generated static_assert messages

source/declarations.tex

@@ -74,10 +74,16 @@
     \opt{attribute-specifier-seq} decl-specifier-seq \opt{ref-qualifier} \terminal{[} identifier-list \terminal{]} initializer \terminal{;}
 \end{bnf}
 
+\begin{bnf}
+\nontermdef{static_assert-message}\br
+  unevaluated-string\br
+  conditional-expression
+\end{bnf}
+
 \begin{bnf}
 \nontermdef{static_assert-declaration}\br
   \keyword{static_assert} \terminal{(} constant-expression \terminal{)} \terminal{;}\br
-  \keyword{static_assert} \terminal{(} constant-expression \terminal{,} unevaluated-string \terminal{)} \terminal{;}
+  \keyword{static_assert} \terminal{(} constant-expression \terminal{,} static_assert-message \terminal{)} \terminal{;}
 \end{bnf}
 
 \begin{bnf}
@@ -250,21 +256,66 @@
 \grammarterm{explicit-specialization}\iref{temp.expl.spec}.
 \end{note}
 
+\pnum
+If a \grammarterm{static_assert-message}
+matches the syntactic requirements of \grammarterm{unevaluated-string},
+it is an \grammarterm{unevaluated-string} and
+the text of the \grammarterm{static_assert-message} is
+the text of the \grammarterm{unevaluated-string}.
+Otherwise, a \grammarterm{static_assert-message} shall be an expression $M$
+such that
+\begin{itemize}
+\item
+the expression \tcode{$M$.size()} is
+implicitly convertible to the type \tcode{std::size_t}, and
+\item
+the expression \tcode{$M$.data()} is
+implicitly convertible to the type ``pointer to \tcode{\keyword{const} \keyword{char}}''.
+\end{itemize}
+
 \pnum
 \indextext{\idxcode{static_assert}}%
 In a \grammarterm{static_assert-declaration},
-the \grammarterm{constant-expression}
+the \grammarterm{constant-expression} $E$
 is contextually converted to \keyword{bool} and
 the converted expression shall be a constant expression\iref{expr.const}.
-If the value of the expression when
-so converted is \tcode{true}
-or the expression is evaluated in the context of a template definition,
-the declaration has no
-effect. Otherwise,
-the \grammarterm{static_assert-declaration} \defnx{fails}{\idxcode{static_assert}!failed},
-the program is ill-formed, and the resulting
-diagnostic message\iref{intro.compliance} should include the text of
-the \grammarterm{unevaluated-string}, if one is supplied.
+If the value of the expression $E$ when so converted is \tcode{true} or
+the expression is evaluated in the context of a template definition,
+the declaration has no effect and
+the \grammarterm{static_assert-message} is
+an unevaluated operand\iref{term.unevaluated.operand}.
+Otherwise,
+the \grammarterm{static_assert-declaration} \defnx{fails}{\idxcode{static_assert}!failed} and
+\begin{itemize}
+\item
+the program is ill-formed, and
+\item
+if the \grammarterm{static_assert-message} is
+a \grammarterm{conditional-expression} $M$,
+\begin{itemize}
+\item
+\tcode{$M$.size()} shall be a converted constant expression of
+type \tcode{std::size_t} and
+let $N$ denote the value of that expression,
+\item
+\tcode{$M$.data()}, implicitly converted to
+the type ``pointer to \tcode{\keyword{const} \keyword{char}}'',
+shall be a core constant expression and let $D$ denote the converted expression,
+\item
+for each $i$ where $0 \le i < N$,
+\tcode{$D$[$i$]} shall be an integral constant expression, and
+\item
+the text of the \grammarterm{static_assert-message} is formed by
+the sequence of $N$ code units, starting at $D$, of
+the ordinary literal encoding\iref{lex.charset}.
+\end{itemize}
+\end{itemize}
+
+\pnum
+\recommended
+When a \grammarterm{static_assert-declaration} fails,
+the resulting diagnostic message should include the text of
+the \grammarterm{static_assert-message}, if one is supplied.
 \begin{example}
 \begin{codeblock}
 static_assert(sizeof(int) == sizeof(void*), "wrong pointer size");

source/preprocessor.tex

@@ -1856,7 +1856,7 @@
 \defnxname{cpp_rvalue_references}                 & \tcode{200610L} \\ \rowsep
 \defnxname{cpp_size_t_suffix}                     & \tcode{202011L} \\ \rowsep
 \defnxname{cpp_sized_deallocation}                & \tcode{201309L} \\ \rowsep
-\defnxname{cpp_static_assert}                     & \tcode{201411L} \\ \rowsep
+\defnxname{cpp_static_assert}                     & \tcode{202306L} \\ \rowsep
 \defnxname{cpp_static_call_operator}              & \tcode{202207L} \\ \rowsep
 \defnxname{cpp_structured_bindings}               & \tcode{201606L} \\ \rowsep
 \defnxname{cpp_template_template_args}            & \tcode{201611L} \\ \rowsep