P0963R3 Structured binding declaration as a condition

Author: jensmaurer

source/declarations.tex

@@ -208,7 +208,8 @@
 \end{example}
 
 \pnum
-A \grammarterm{simple-declaration} with a \grammarterm{structured-binding-declaration} is called
+A \grammarterm{simple-declaration} or a \grammarterm{condition}
+with a \grammarterm{structured-binding-declaration} is called
 a \defn{structured binding declaration}\iref{dcl.struct.bind}.
 Each \grammarterm{decl-specifier} in the \grammarterm{decl-specifier-seq}
 shall be
@@ -227,9 +228,13 @@
 of the form ``\tcode{=} \grammarterm{assignment-expression}'',
 of the form ``\tcode{\{} \grammarterm{assignment-expression} \tcode{\}}'',
 or
-of the form ``\tcode{(} \grammarterm{assignment-expression} \tcode{)}'',
-where the
-\grammarterm{assignment-expression} is of array or non-union class type.
+of the form ``\tcode{(} \grammarterm{assignment-expression} \tcode{)}''.
+If the \grammarterm{structured-binding-declaration} appears as
+a \grammarterm{condition},
+the \grammarterm{assignment-expression} shall be of non-union class type.
+Otherwise,
+the \grammarterm{assignment-expression} shall be of
+array or non-union class type.
 
 \pnum
 If the \grammarterm{decl-specifier-seq} contains the \keyword{typedef}
@@ -7042,6 +7047,10 @@
 \tcode{E} is never a reference type\iref{expr.prop}.
 \end{note}
 
+\pnum
+If a structured binding declaration appears as a \grammarterm{condition},
+the decision variable\iref{stmt.pre} of the condition is \tcode{e}.
+
 \pnum
 If the \grammarterm{initializer} refers to
 one of the names introduced by the structured binding declaration,
@@ -7105,6 +7114,11 @@
 Each $\tcode{v}_i$ is the name of an lvalue of type $\tcode{T}_i$
 that refers to the object bound to $\tcode{r}_i$;
 the referenced type is $\tcode{T}_i$.
+The initialization of \tcode{e} and
+any conversion of \tcode{e} considered as a decision variable\iref{stmt.pre}
+is sequenced before the initialization of any $\tcode{r}_i$.
+The initialization of $\tcode{r}_i$ is sequenced before
+the initialization of $\tcode{r}_j$ if $i < j$.
 
 \pnum
 Otherwise,

source/statements.tex

@@ -34,6 +34,7 @@
 \nontermdef{condition}\br
     expression\br
     \opt{attribute-specifier-seq} decl-specifier-seq declarator brace-or-equal-initializer
+    structured-binding-declaration initializer
 \end{bnf}
 
 The optional \grammarterm{attribute-specifier-seq} appertains to the respective statement.
@@ -86,18 +87,29 @@
 The rules for \grammarterm{condition}{s} apply both
 to \grammarterm{selection-statement}{s}\iref{stmt.select} and
 to the \keyword{for} and \keyword{while} statements\iref{stmt.iter}.
-A \grammarterm{condition} that is not an \grammarterm{expression} is a
-declaration\iref{dcl.dcl}.
+If a \grammarterm{structured-binding-declaration}
+appears in a \grammarterm{condition},
+the \grammarterm{condition} is a structured binding declaration\iref{dcl.pre}.
+A \grammarterm{condition} that is
+neither an \grammarterm{expression} nor a structured binding declaration
+is a declaration\iref{dcl.dcl}.
 The \grammarterm{declarator} shall not
 specify a function or an array. The \grammarterm{decl-specifier-seq} shall not
 define a class or enumeration. If the \keyword{auto} \grammarterm{type-specifier} appears in
 the \grammarterm{decl-specifier-seq},
 the type of the identifier being declared is deduced from the initializer as described in~\ref{dcl.spec.auto}.
 
 \pnum
-The value of a \grammarterm{condition} that is an initialized declaration
+The \defn{decision variable} of a \grammarterm{condition}
+that is neither an \grammarterm{expression} nor a structured binding declaration
+is the declared variable.
+The decision variable of a \grammarterm{condition}
+that is a structured binding declaration is specified in\iref{dcl.struct.bind}.
+
+\pnum
+The value of a \grammarterm{condition} that not an \grammarterm{expression}
 in a statement other than a \keyword{switch} statement is the value of the
-declared variable
+decision variable
 contextually converted to \tcode{bool}\iref{conv}.
 If that
 conversion is ill-formed, the program is ill-formed.
@@ -116,7 +128,10 @@
 it is interpreted as the latter.
 
 \pnum
-In the \grammarterm{decl-specifier-seq} of a \grammarterm{condition}, each
+In the \grammarterm{decl-specifier-seq} of a \grammarterm{condition},
+including that of any \grammarterm{structured-binding-declaration} of
+the \grammarterm{condition},
+each
 \grammarterm{decl-specifier} shall be either a \grammarterm{type-specifier}
 or \keyword{constexpr}.
 
@@ -408,10 +423,9 @@
 several statements depending on the value of a condition.
 
 \pnum
-The value of a \grammarterm{condition}
-that is an initialized declaration
-is the value of the declared variable,
-or the value of the \grammarterm{expression} otherwise.
+If the \grammarterm{condition} is an \grammarterm{expression},
+the value of the condition is the value of the \grammarterm{expression};
+otherwise, it is the value of the decision variable.
 The value of the condition shall be of integral type, enumeration type, or class
 type. If of class type, the
 condition is contextually implicitly converted\iref{conv} to