CWG3140 Allowing expansion over non-constant std::array

tkoeppe · Apr 12, 2026 · Apr 5, 2026 · Apr 5, 2026 · Apr 5, 2026 · Apr 5, 2026
commit 0d2ab00dfd2b13d7461f01e695b7c1f20021478e
diff --git a/source/basic.tex b/source/basic.tex
@@ -4977,7 +4977,7 @@
 \pnum
 The sixth context is when a temporary object is created
 in the \grammarterm{expansion-initializer}
-of a destructuring expansion statement.
+of an iterating or destructuring expansion statement.
 If such a temporary object would otherwise be destroyed
 at the end of that \grammarterm{expansion-initializer},
 the object persists for the lifetime of the reference

diff --git a/source/statements.tex b/source/statements.tex
@@ -874,41 +874,43 @@
 \begin{codeblock}
 {
   @\grammarterm{init-statement}@
-  constexpr decltype(auto) @\exposidnc{range}@ = (@\grammarterm{expansion-initializer}@);
-  constexpr auto @\exposidnc{begin}@ = @\exposidnc{begin-expr}@;            // see \ref{stmt.ranged}
-  constexpr auto @\exposidnc{end}@ = @\exposidnc{end-expr}@;                // see \ref{stmt.ranged}
-
+  @\opt{constexpr}@ decltype(auto) @\exposidnc{range}@ = (@\grammarterm{expansion-initializer}@);
+  @\opt{constexpr}@ auto @\exposidnc{begin}@ = @\exposidnc{begin-expr}@;         // see \ref{stmt.ranged}
   @$S_{0}$@
   @\vdots@
   @$S_{N-1}$@
 }
 \end{codeblock}
 where $N$ is the result of evaluating the expression
 \begin{codeblock}
-[] consteval {
+[&] consteval {
   std::ptrdiff_t result = 0;
-  for (auto i = @\exposid{begin}@; i != @\exposid{end}@; ++i) ++result;
+  auto b = @\exposid{begin-expr}@;
+  auto e = @\exposid{end-expr}@;
+  for (; b != e; ++b) ++result;
   return result;                                // distance from \exposid{begin} to \exposid{end}
 }()
 \end{codeblock}
 and $S_{i}$ is
 \begin{codeblock}
 {
-  constexpr auto @\exposid{iter}@ = @\exposid{begin}@ + decltype(begin - begin){@\placeholder{i}@};
+  @\opt{constexpr}@ auto @\exposid{iter}@ = @\exposid{begin}@ + decltype(begin - begin){@\placeholder{i}@};
   @\grammarterm{for-range-declaration}@ = *@\exposid{iter}@;
   @\grammarterm{compound-statement}@
 }
 \end{codeblock}
-The variables \exposid{range}, \exposid{begin}, \exposid{end}, and \exposid{iter}
+The variables \exposid{range}, \exposid{begin}, and \exposid{iter}
 are defined for exposition only.
+The keyword \keyword{constexpr} is present in the declarations
+of \exposid{range}, \exposid{begin}, and \exposid{iter}
+if and only if
+\keyword{constexpr} is one of the \grammarterm{decl-specifier}{s} of
+the \grammarterm{decl-specifier-seq} of
+the \grammarterm{for-range-declaration}.
 The identifier \tcode{\placeholder{i}} is considered to be
 a prvalue of type \tcode{std::ptrdiff_t};
 the program is ill-formed if
 \tcode{\placeholder{i}} is not representable as such a value.
-\begin{note}
-The instantiation is ill-formed if \exposid{range}
-is not a constant expression\iref{expr.const.const}.
-\end{note}
 
 \item
 Otherwise, $S$ is a destructuring expansion statement and,