P2686R5 constexpr structured bindings and references to constexpr variables

Editorial notes:
 * Merge the change to [temp.arg.nontype] with existing text.

Author: jensmaurer

source/basic.tex

@@ -441,24 +441,59 @@
 A variable is named by an expression
 if the expression is an \grammarterm{id-expression} that denotes it.
 A variable \tcode{x} that is named by a
-potentially-evaluated expression $E$
-is \defnx{odr-used}{odr-use} by $E$ unless
+potentially-evaluated expression $N$
+that appears at a point $P$
+is \defnx{odr-used}{odr-use} by $N$ unless
 \begin{itemize}
 \item
-  \tcode{x} is a reference that is
-  usable in constant expressions\iref{expr.const}, or
+\tcode{x} is a reference
+that is usable in constant expressions at $P$\iref{expr.const} or
 \item
-  \tcode{x} is a variable of non-reference type that is
-  usable in constant expressions and has no mutable subobjects, and
-  $E$ is an element of the set of potential results of an expression
-  of non-volatile-qualified non-class type
-  to which the lvalue-to-rvalue conversion\iref{conv.lval} is applied, or
+$N$ is an element of the set of potential results of an expression $E$, where
+\begin{itemize}
+\item
+$E$ is a discarded-value expression\iref{expr.context}
+to which the lvalue-to-rvalue conversion is not applied or
+\item
+\tcode{x} is a non-volatile object
+that is usable in constant expressions at $P$ and
+has no mutable subobjects and
+\begin{itemize}
+\item
+$E$ is a class member access expression\iref{expr.ref}
+naming a non-static data member of reference type and
+whose object expression has non-volatile-qualified type or
 \item
-  \tcode{x} is a variable of non-reference type, and
-  $E$ is an element of the set of potential results
-  of a discarded-value expression\iref{expr.context}
-  to which the lvalue-to-rvalue conversion is not applied.
+the lvalue-to-rvalue conversion\iref{conv.lval} is applied to $E$ and
+$E$ has non-volatile-qualified non-class type
 \end{itemize}
+\end{itemize}
+\end{itemize}
+\begin{example}
+\begin{codeblock}
+int f(int);
+int g(int&);
+struct A {
+  int x;
+};
+struct B {
+  int& r;
+};
+int h(bool cond) {
+  constexpr A a = {1};
+  constexpr const volatile A& r = a;    // odr-uses \tcode{a}
+  int _ = f(cond ? a.x : r.x);          // does not odr-use \tcode{a} or \tcode{r}
+  int x, y;
+  constexpr B b1 = {x}, b2 = {y};       // odr-uses \tcode{x} and \tcode{y}
+  int _ = g(cond ? b1.r : b2.r);        // does not odr-use \tcode{b1} or \tcode{b2}
+  int _ = ((cond ? x : y), 0);          // does not odr-use \tcode{x} or \tcode{y}
+  return [] {
+    return b1.r;                        // error: \tcode{b1} is odr-used here because the object
+                                        // referred to by \tcode{b1.r} is not constexpr-referenceable here
+  }();
+}
+\end{codeblock}
+\end{example}
 
 \pnum
 A structured binding is odr-used if it appears as a potentially-evaluated expression.
@@ -6926,7 +6961,7 @@
 \pnum
 \indextext{initialization!constant}%
 \defnx{Constant initialization}{constant initialization} is performed
-if a variable or temporary object with static or thread storage duration
+if a variable with static or thread storage duration
 is constant-initialized\iref{expr.const}.
 \indextext{initialization!zero-initialization}%
 If constant initialization is not performed, a variable with static

source/declarations.tex

@@ -212,6 +212,8 @@
 a \defn{structured binding declaration}\iref{dcl.struct.bind}.
 Each \grammarterm{decl-specifier} in the \grammarterm{decl-specifier-seq}
 shall be
+\tcode{constexpr},
+\tcode{constinit},
 \tcode{static},
 \tcode{thread_local},
 \tcode{auto}\iref{dcl.spec.auto}, or
@@ -849,7 +851,8 @@
 
 \pnum
 The \keyword{constexpr} specifier shall be applied only to
-the definition of a variable or variable template or
+the definition of a variable or variable template,
+a structured binding declaration, or
 the declaration of a function or function template.
 The \keyword{consteval} specifier shall be applied only to
 the declaration of a function or function template.
@@ -994,9 +997,7 @@
 Such an object
 shall have literal type and
 shall be initialized.
-In any \keyword{constexpr} variable declaration,
-the full-expression of the initialization
-shall be a constant expression\iref{expr.const}.
+A \keyword{constexpr} variable shall be constant-initializable\iref{expr.const}.
 A \keyword{constexpr} variable that is an object,
 as well as any temporary to which a \keyword{constexpr} reference is bound,
 shall have constant destruction.
@@ -1007,6 +1008,16 @@
 };
 constexpr pixel ur = { 1294, 1024 };    // OK
 constexpr pixel origin;                 // error: initializer missing
+
+namespace N {
+  void f() {
+    int x;
+    constexpr int& ar = x;              // OK
+    static constexpr int& sr = x;       // error: \tcode{x} is not constexpr-representable
+                                        // at the point indicated below
+  }
+  // immediate scope here is that of \tcode{N}
+}
 \end{codeblock}
 \end{example}
 
@@ -1015,7 +1026,12 @@
 
 \pnum
 The \keyword{constinit} specifier shall be applied only
-to a declaration of a variable with static or thread storage duration.
+to a declaration of a variable with static or thread storage duration
+or to a structured binding declaration\iref{dcl.struct.bind}.
+\begin{note}
+A structured binding declaration introduces a uniquely named variable,
+to which the \tcode{constinit} specifier applies.
+\end{note}
 If the specifier is applied to any declaration of a variable,
 it shall be applied to the initializing declaration.
 No diagnostic is required if no \keyword{constinit} declaration
@@ -7033,8 +7049,10 @@
 appertains to the structured binding so introduced.
 Let \cv{} denote the \grammarterm{cv-qualifier}{s} in
 the \grammarterm{decl-specifier-seq} and
-\placeholder{S} consist of the \grammarterm{storage-class-specifier}{s} of
-the \grammarterm{decl-specifier-seq} (if any).
+\placeholder{S} consist of
+each \grammarterm{decl-specifier} of the \grammarterm{decl-specifier-seq}
+that is \tcode{constexpr}, \tcode{constinit}, or
+a \grammarterm{storage-class-specifier}.
 A \cv{} that includes \tcode{volatile} is deprecated;
 see~\ref{depr.volatile.type}.
 First, a variable with a unique name \exposid{e} is introduced. If the