Merge 2022-07 CWG Motion 7

P2324R2 Labels at the end of compound statements (C compatibility)

Author: tkoeppe

source/statements.tex

@@ -117,19 +117,25 @@
 \grammarterm{decl-specifier} shall be either a \grammarterm{type-specifier}
 or \keyword{constexpr}.
 
-\rSec1[stmt.label]{Labeled statement}%
+\rSec1[stmt.label]{Label}%
 \indextext{statement!labeled}
 
 \pnum
 \indextext{statement!labeled}%
 \indextext{\idxcode{:}!label specifier}%
-A statement can be labeled.
+A label can be added to a statement or
+used anywhere in a \grammarterm{compound-statement}.
+
+\begin{bnf}
+\nontermdef{label}\br
+    \opt{attribute-specifier-seq} identifier \terminal{:}\br
+    \opt{attribute-specifier-seq} \keyword{case} constant-expression \terminal{:}\br
+    \opt{attribute-specifier-seq} \keyword{default} \terminal{:}
+\end{bnf}
 
 \begin{bnf}
 \nontermdef{labeled-statement}\br
-    \opt{attribute-specifier-seq} identifier \terminal{:} statement\br
-    \opt{attribute-specifier-seq} \keyword{case} constant-expression \terminal{:} statement\br
-    \opt{attribute-specifier-seq} \keyword{default} \terminal{:} statement
+    label statement
 \end{bnf}
 
 The optional \grammarterm{attribute-specifier-seq} appertains to the label.
@@ -139,7 +145,7 @@
 \indextext{label!scope of}%
 No two labels in a function shall have the same \grammarterm{identifier}.
 A label can be used in a \tcode{goto} statement
-before its introduction by a \grammarterm{labeled-statement}.
+before its introduction.
 
 \pnum
 \indextext{label!\idxcode{case}}%
@@ -169,8 +175,7 @@
 a \defn{null statement}.
 \begin{note}
 Most statements are expression statements --- usually assignments or
-function calls. A null statement is useful to carry a label just before
-the \tcode{\}} of a compound statement and to supply a null body to an
+function calls. A null statement is useful to supply a null body to an
 iteration statement such as a \keyword{while}
 statement\iref{stmt.while}.
 \end{note}
@@ -184,7 +189,7 @@
 
 \begin{bnf}
 \nontermdef{compound-statement}\br
-    \terminal{\{} \opt{statement-seq} \terminal{\}}
+    \terminal{\{} \opt{statement-seq} \opt{label-seq} \terminal{\}}
 \end{bnf}
 
 \begin{bnf}
@@ -193,6 +198,16 @@
     statement-seq statement
 \end{bnf}
 
+\begin{bnf}
+\nontermdef{label-seq}\br
+    label\br
+    label-seq label
+\end{bnf}
+
+A label at the end of a \grammarterm{compound-statement}
+is treated as if it were followed by a null statement.
+
+\pnum
 \begin{note}
 A compound statement defines a block scope\iref{basic.scope}.
 A declaration is a \grammarterm{statement}\iref{stmt.dcl}.