c++: Implement part of C++26 P2686R4 - constexpr structured bindings [PR117784]

The following patch implements the constexpr structured bindings part of
the P2686R4 paper, so the [dcl.pre], [dcl.struct.bind], [dcl.constinit]
and first hunk in [dcl.constexpr] changes.
The paper doesn't have a feature test macro and the constexpr structured
binding part of it seems more-less self-contained, so I think it is useful
to get this in independently from the rest.
Of course, automatic constexpr/constinit structured bindings in the
tuple cases or automatic constexpr/constinit structured bindings with auto &
will not really work for now.
Another reason for the split is that for C++ < 26, I think what the patch
implements is basically what the users will see, i.e. we can accept
constexpr or constinit structured binding with pedwarn, but I think we can't
change the constant expression rules in C++ < 26.

I plan to look at the rest of the paper.

2025-07-08  Jakub Jelinek  <jakub@redhat.com>

	PR c++/117784
	* decl.cc: Implement part of C++26 P2686R4 - constexpr structured
	bindings.
	(cp_finish_decl): Pedwarn for C++23 and older on constinit on
	structured bindings except for static/thread_local where it uses
	earlier error.
	(grokdeclarator): Pedwarn on constexpr structured bindings for
	C++23 and older instead of emitting error always, don't clear
	constexpr_p in that case.
	* parser.cc (cp_parser_decomposition_declaration): Copy over
	DECL_DECLARED_CONSTEXPR_P and DECL_DECLARED_CONSTINIT_P flags.

	* g++.dg/cpp1z/decomp3.C (test): For constexpr structured binding
	initialize from constexpr var instead of non-constexpr and expect
	just a pedwarn for C++23 and older instead of error always.
	* g++.dg/cpp26/decomp9.C (foo): Likewise.
	* g++.dg/cpp26/decomp22.C: New test.
	* g++.dg/cpp26/decomp23.C: New test.
	* g++.dg/cpp26/decomp24.C: New test.
	* g++.dg/cpp26/decomp25.C: New test.

8 files changed, 305 insertions, 10 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index be26bd3..99b9854 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -9174,6 +9174,10 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 
       if (decomp)
 	{
+	  if (DECL_DECLARED_CONSTINIT_P (decl) && cxx_dialect < cxx26)
+	    pedwarn (DECL_SOURCE_LOCATION (decl), OPT_Wc__26_extensions,
+		     "%<constinit%> can be applied to structured binding "
+		     "only with %<-std=c++2c%> or %<-std=gnu++2c%>");
 	  cp_maybe_mangle_decomp (decl, decomp);
 	  if (TREE_STATIC (decl) && !DECL_FUNCTION_SCOPE_P (decl))
 	    {
@@ -13621,9 +13625,10 @@ grokdeclarator (const cp_declarator *declarator,
       if (typedef_p)
 	error_at (declspecs->locations[ds_typedef],
 		  "structured binding declaration cannot be %qs", "typedef");
-      if (constexpr_p && !concept_p)
-	error_at (declspecs->locations[ds_constexpr], "structured "
-		  "binding declaration cannot be %qs", "constexpr");
+      if (constexpr_p && !concept_p && cxx_dialect < cxx26)
+	pedwarn (declspecs->locations[ds_constexpr], OPT_Wc__26_extensions,
+		 "structured binding declaration can be %qs only with "
+		 "%<-std=c++2c%> or %<-std=gnu++2c%>", "constexpr");
       if (consteval_p)
 	error_at (declspecs->locations[ds_consteval], "structured "
 		  "binding declaration cannot be %qs", "consteval");
@@ -13634,8 +13639,11 @@ grokdeclarator (const cp_declarator *declarator,
 		 declspecs->gnu_thread_keyword_p
 		 ? "__thread" : "thread_local");
       if (concept_p)
-	error_at (declspecs->locations[ds_concept],
-		  "structured binding declaration cannot be %qs", "concept");
+	{
+	  error_at (declspecs->locations[ds_concept],
+		    "structured binding declaration cannot be %qs", "concept");
+	  constexpr_p = 0;
+	}
       /* [dcl.struct.bind] "A cv that includes volatile is deprecated."  */
       if (type_quals & TYPE_QUAL_VOLATILE)
 	warning_at (declspecs->locations[ds_volatile], OPT_Wvolatile,
@@ -13690,7 +13698,6 @@ grokdeclarator (const cp_declarator *declarator,
 		 "%<auto%> type %qT", type);
       inlinep = 0;
       typedef_p = 0;
-      constexpr_p = 0;
       consteval_p = 0;
       concept_p = 0;
       if (storage_class != sc_static)
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 239e6f9..32c6a42 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -16870,7 +16870,11 @@ cp_parser_decomposition_declaration (cp_parser *parser,
 	  decl = error_mark_node;
 	}
       else
-	prev = decl2;
+	{
+	  prev = decl2;
+	  DECL_DECLARED_CONSTEXPR_P (decl2) = DECL_DECLARED_CONSTEXPR_P (decl);
+	  DECL_DECLARED_CONSTINIT_P (decl2) = DECL_DECLARED_CONSTINIT_P (decl);
+	}
       if (elt_pushed_scope)
 	pop_scope (elt_pushed_scope);
     }
diff --git a/gcc/testsuite/g++.dg/cpp1z/decomp3.C b/gcc/testsuite/g++.dg/cpp1z/decomp3.C
index a9b23b0..202edc3 100644
--- a/gcc/testsuite/g++.dg/cpp1z/decomp3.C
+++ b/gcc/testsuite/g++.dg/cpp1z/decomp3.C
@@ -19,7 +19,8 @@ test (A &b, B c)
 					// { dg-error "expected primary-expression before 'decltype'" "" { target c++11_down } .-2 }
   auto & & && & [ m, n, o ] = b;	// { dg-error "multiple ref-qualifiers" }
 					// { dg-warning "structured bindings only available with '-std=c..17' or '-std=gnu..17'" "" { target c++14_down } .-1 }
-  constexpr auto [ p ] = c;		// { dg-error "structured binding declaration cannot be 'constexpr'" }
+  constexpr B c2 = { 42 };
+  constexpr auto [ p ] = c2;		// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } }
 					// { dg-warning "structured bindings only available with '-std=c..17' or '-std=gnu..17'" "" { target c++14_down } .-1 }
   friend auto [ q ] = c;		// { dg-error "'friend' used outside of class" }
 					// { dg-warning "structured bindings only available with '-std=c..17' or '-std=gnu..17'" "" { target c++14_down } .-1 }
diff --git a/gcc/testsuite/g++.dg/cpp26/decomp22.C b/gcc/testsuite/g++.dg/cpp26/decomp22.C
new file mode 100644
index 0000000..576a93b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/decomp22.C
@@ -0,0 +1,66 @@
+// C++26 P2686R4 - constexpr structured bindings
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+namespace std {
+  template <typename T> struct tuple_size;
+  template <int, typename> struct tuple_element;
+}
+
+struct A {
+  int i, j;
+  template <int I> constexpr const int &get () const { return I == 1 ? j : i; }
+};
+
+template <> struct std::tuple_size <const A> { static const int value = 3; };
+template <int I> struct std::tuple_element <I, const A> { using type = const int; };
+
+constexpr struct B {
+  int i, j;
+  long long k, l;
+} a[3] = { { 1, 2, 3, 4 }, { 5, 6, 7, 8 }, { 9, 10, 11, 12 } };
+
+constexpr auto [ b, c, d ] = a;				// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+#if __cpp_constinit >= 201907
+constinit auto [ e, f, g ] = a;				// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+constexpr auto [ h, i, j, k ] = a[1];			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+#if __cpp_constinit >= 201907
+constinit auto [ l, m, n, o ] = a[2];			// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+constexpr auto & [ p, q, r ] = a;			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+#if __cpp_constinit >= 201907
+constinit auto & [ s, t, u ] = a;			// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+constexpr auto & [ v, w, x, y ] = a[1];			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+#if __cpp_constinit >= 201907
+constinit auto & [ aa, ab, ac, ad ] = a[2];		// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+static_assert (b.i == 1 && b.l == 4 && c.j == 6 && c.k == 7 && d.i == 9 && d.k == 11, "");
+static_assert (h == 5 && i == 6 && j == 7 && k == 8, "");
+static_assert (p.i == 1 && p.l == 4 && q.j == 6 && q.k == 7 && r.i == 9 && r.k == 11, "");
+static_assert (&p.i == &a[0].i && &p.l == &a[0].l && &q.j == &a[1].j, "");
+static_assert (&q.k == &a[1].k && &r.i == &a[2].i && &r.k == &a[2].k, "");
+static_assert (v == 5 && w == 6 && x == 7 && y == 8, "");
+static_assert (&v == &a[1].i && &w == &a[1].j && &x == &a[1].k && &y == &a[1].l, "");
+
+constexpr A z = { 42, -42 };
+constexpr auto [ ae, af, ag ] = z;			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+#if __cpp_constinit >= 201907
+constinit const auto [ ah, ai, aj ] = z;		// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+constexpr auto & [ ak, al, am ] = z;			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+#if __cpp_constinit >= 201907
+constinit auto & [ an, ao, ap ] = z;			// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+static_assert (ae == 42 && af == -42 && ag == 42, "");
+static_assert (&af == &ae + 1 && &ag == &ae, "");
+static_assert (&ae != &z.i && &af != &z.j && &ag != &z.i, "");
+static_assert (ak == 42 && al == -42 && am == 42, "");
+static_assert (&ak == &z.i && &al == &z.j && &am == &z.i, "");
diff --git a/gcc/testsuite/g++.dg/cpp26/decomp23.C b/gcc/testsuite/g++.dg/cpp26/decomp23.C
new file mode 100644
index 0000000..ad2f7e0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/decomp23.C
@@ -0,0 +1,77 @@
+// C++26 P2686R4 - constexpr structured bindings
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+namespace std {
+  template <typename T> struct tuple_size;
+  template <int, typename> struct tuple_element;
+}
+
+struct A {
+  int i, j;
+  template <int I> constexpr const int &get () const { return I == 1 ? j : i; }
+};
+
+template <> struct std::tuple_size <const A> { static const int value = 3; };
+template <int I> struct std::tuple_element <I, const A> { using type = const int; };
+
+struct B {
+  int i, j;
+  long long k, l;
+};
+
+void
+foo ()
+{
+  static constexpr B a[3] = { { 1, 2, 3, 4 }, { 5, 6, 7, 8 }, { 9, 10, 11, 12 } };
+  static constexpr auto [ b, c, d ] = a;		// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-2 }
+#if __cpp_constinit >= 201907
+  static constinit auto [ e, f, g ] = a;		// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+  static constexpr auto [ h, i, j, k ] = a[1];		// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-2 }
+#if __cpp_constinit >= 201907
+  static constinit auto [ l, m, n, o ] = a[2];		// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+  static constexpr auto & [ p, q, r ] = a;		// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-2 }
+#if __cpp_constinit >= 201907
+  static constinit auto & [ s, t, u ] = a;		// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+  static constexpr auto & [ v, w, x, y ] = a[1];	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-2 }
+#if __cpp_constinit >= 201907
+  static constinit auto & [ aa, ab, ac, ad ] = a[2];	// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+  static_assert (b.i == 1 && b.l == 4 && c.j == 6 && c.k == 7 && d.i == 9 && d.k == 11, "");
+  static_assert (h == 5 && i == 6 && j == 7 && k == 8, "");
+  static_assert (p.i == 1 && p.l == 4 && q.j == 6 && q.k == 7 && r.i == 9 && r.k == 11, "");
+  static_assert (&p.i == &a[0].i && &p.l == &a[0].l && &q.j == &a[1].j, "");
+  static_assert (&q.k == &a[1].k && &r.i == &a[2].i && &r.k == &a[2].k, "");
+  static_assert (v == 5 && w == 6 && x == 7 && y == 8, "");
+  static_assert (&v == &a[1].i && &w == &a[1].j && &x == &a[1].k && &y == &a[1].l, "");
+
+  static constexpr A z = { 42, -42 };
+  static constexpr auto [ ae, af, ag ] = z;		// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-2 }
+#if __cpp_constinit >= 201907
+  static constinit const auto [ ah, ai, aj ] = z;	// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+  static constexpr auto & [ ak, al, am ] = z;		// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-2 }
+#if __cpp_constinit >= 201907
+  static constinit auto & [ an, ao, ap ] = z;		// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+  static_assert (ae == 42 && af == -42 && ag == 42, "");
+  static_assert (&af == &ae + 1 && &ag == &ae, "");
+  static_assert (&ae != &z.i && &af != &z.j && &ag != &z.i, "");
+  static_assert (ak == 42 && al == -42 && am == 42, "");
+  static_assert (&ak == &z.i && &al == &z.j && &am == &z.i, "");
+}
diff --git a/gcc/testsuite/g++.dg/cpp26/decomp24.C b/gcc/testsuite/g++.dg/cpp26/decomp24.C
new file mode 100644
index 0000000..5da1321
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/decomp24.C
@@ -0,0 +1,20 @@
+// C++26 P2686R4 - constexpr structured bindings
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+struct B {
+  int i, j;
+  long long k, l;
+};
+
+void
+foo ()
+{
+  constexpr B a[3] = { { 1, 2, 3, 4 }, { 5, 6, 7, 8 }, { 9, 10, 11, 12 } };
+  constexpr auto [ b, c, d ] = a;			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+  constexpr auto [ h, i, j, k ] = a[1];			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+  static_assert (b.i == 1 && b.l == 4 && c.j == 6 && c.k == 7 && d.i == 9 && d.k == 11, "");
+  static_assert (h == 5 && i == 6 && j == 7 && k == 8, "");
+}
diff --git a/gcc/testsuite/g++.dg/cpp26/decomp25.C b/gcc/testsuite/g++.dg/cpp26/decomp25.C
new file mode 100644
index 0000000..55559f0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/decomp25.C
@@ -0,0 +1,119 @@
+// C++26 P2686R4 - constexpr structured bindings
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+namespace std {
+  template <typename T> struct tuple_size;
+  template <int, typename> struct tuple_element;
+}
+
+struct A {
+  int i, j;
+  template <int I> int &get () { return I == 1 ? j : i; }
+};
+
+template <> struct std::tuple_size <A> { static const int value = 3; };
+template <int I> struct std::tuple_element <I, A> { using type = int; };
+template <> struct std::tuple_size <const A> { static const int value = 3; };
+template <int I> struct std::tuple_element <I, const A> { using type = int; };
+
+struct B {
+  int i, j;
+  long long k, l;
+} a[3] = { { 1, 2, 3, 4 }, { 5, 6, 7, 8 }, { 9, 10, 11, 12 } };	// { dg-message "'a' was not declared 'constexpr'" "" }
+
+struct C {
+  int i, j;
+  template <int I> const int &get () const { return I == 1 ? j : i; }
+};
+
+template <> struct std::tuple_size <const C> { static const int value = 3; };
+template <int I> struct std::tuple_element <I, const C> { using type = const int; };
+
+constexpr auto [ b, c, d ] = a;				// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-error "the value of 'a' is not usable in a constant expression" "" { target *-*-* } .-2 }
+#if __cpp_constinit >= 201907
+constinit auto [ e, f, g ] = a;				// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+							// { dg-error "'constinit' variable '<structured bindings>' does not have a constant initializer" "" { target c++20 } .-1 }
+							// { dg-error "the value of 'a' is not usable in a constant expression" "" { target c++20 } .-2 }
+#endif
+constexpr auto [ h, i, j, k ] = a[1];			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-error "the value of 'a' is not usable in a constant expression" "" { target *-*-* } .-2 }
+#if __cpp_constinit >= 201907
+constinit auto [ l, m, n, o ] = a[2];			// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+							// { dg-error "'constinit' variable '<structured bindings>' does not have a constant initializer" "" { target c++20 } .-1 }
+							// { dg-error "the value of 'a' is not usable in a constant expression" "" { target c++20 } .-2 }
+#endif
+constexpr auto & [ p, q, r ] = a;			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+#if __cpp_constinit >= 201907
+constinit auto & [ s, t, u ] = a;			// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+constexpr auto & [ v, w, x, y ] = a[1];			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+#if __cpp_constinit >= 201907
+constinit auto & [ aa, ab, ac, ad ] = a[2];		// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+#endif
+
+A z = { 42, -42 };					// { dg-message "'z' was not declared 'constexpr'" "" }
+constexpr auto [ ae, af, ag ] = z;			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-error "the value of 'z' is not usable in a constant expression" "" { target *-*-* } .-2 }
+							// { dg-error "passing 'const A' as 'this' argument discards qualifiers" "" { target *-*-* } .-3 }
+							// { dg-error "call to non-'constexpr' function 'int\\\& A::get\\\(\\\)" "" { target *-*-* } .-4 }
+#if __cpp_constinit >= 201907
+constinit const auto [ ah, ai, aj ] = z;		// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+							// { dg-error "'constinit' variable '<structured bindings>' does not have a constant initializer" "" { target c++20 } .-1 }
+							// { dg-error "the value of 'z' is not usable in a constant expression" "" { target c++20 } .-2 }
+							// { dg-error "passing 'const A' as 'this' argument discards qualifiers" "" { target c++20 } .-3 }
+							// { dg-error "call to non-'constexpr' function 'int\\\& A::get\\\(\\\)" "" { target c++20 } .-4 }
+							// { dg-error "'constinit' variable 'ah' does not have a constant initializer" "" { target c++20 } .-5 }
+							// { dg-error "'constinit' variable 'ai' does not have a constant initializer" "" { target c++20 } .-6 }
+							// { dg-error "'constinit' variable 'aj' does not have a constant initializer" "" { target c++20 } .-7 }
+#endif
+constexpr auto & [ ak, al, am ] = z;			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-error "call to non-'constexpr' function 'int\\\& A::get\\\(\\\)" "" { target *-*-* } .-2 }
+#if __cpp_constinit >= 201907
+constinit auto & [ an, ao, ap ] = z;			// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+							// { dg-error "'constinit' variable 'an' does not have a constant initializer" "" { target c++20 } .-1 }
+							// { dg-error "'constinit' variable 'ao' does not have a constant initializer" "" { target c++20 } .-2 }
+							// { dg-error "'constinit' variable 'ap' does not have a constant initializer" "" { target c++20 } .-3 }
+							// { dg-message "call to non-'constexpr' function 'int\\\& A::get\\\(\\\)" "" { target c++20 } .-4 }
+#endif
+
+constexpr C zz = { 42, -42 };
+constexpr auto [ aq, ar, as ] = zz;			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-error "call to non-'constexpr' function 'const int\\\& C::get\\\(\\\) const" "" { target *-*-* } .-2 }
+#if __cpp_constinit >= 201907
+constinit const auto [ at, au, av ] = zz;		// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+							// { dg-error "call to non-'constexpr' function 'const int\\\& C::get\\\(\\\) const" "" { target c++20 } .-1 }
+							// { dg-error "'constinit' variable 'at' does not have a constant initializer" "" { target c++20 } .-2 }
+							// { dg-error "'constinit' variable 'au' does not have a constant initializer" "" { target c++20 } .-3 }
+							// { dg-error "'constinit' variable 'av' does not have a constant initializer" "" { target c++20 } .-4 }
+#endif
+constexpr auto & [ aw, ax, ay ] = zz;			// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+							// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+							// { dg-error "call to non-'constexpr' function 'const int\\\& C::get\\\(\\\) const" "" { target *-*-* } .-2 }
+#if __cpp_constinit >= 201907
+constinit auto & [ az, ba, bb ] = zz;			// { dg-warning "'constinit' can be applied to structured binding only with" "" { target { c++20 && c++23_down } } }
+							// { dg-error "'constinit' variable 'az' does not have a constant initializer" "" { target c++20 } .-1 }
+							// { dg-error "'constinit' variable 'ba' does not have a constant initializer" "" { target c++20 } .-2 }
+							// { dg-error "'constinit' variable 'bb' does not have a constant initializer" "" { target c++20 } .-3 }
+							// { dg-message "call to non-'constexpr' function 'const int\\\& C::get\\\(\\\) const" "" { target c++20 } .-4 }
+#endif
+
+void
+foo ()
+{
+#if __cpp_constinit >= 201907
+  constexpr B a[3] = { { 1, 2, 3, 4 }, { 5, 6, 7, 8 }, { 9, 10, 11, 12 } };
+  constinit auto [ b, c, d ] = a;			// { dg-error "'constinit' can only be applied to a variable with static or thread storage duration" "" { target c++20 } }
+  constinit auto & [ e, f, g ] = a;			// { dg-error "'constinit' can only be applied to a variable with static or thread storage duration" "" { target c++20 } }
+  constinit auto [ h, i, j, k ] = a[1];			// { dg-error "'constinit' can only be applied to a variable with static or thread storage duration" "" { target c++20 } }
+  constinit auto & [ l, m, n, o ] = a[2];		// { dg-error "'constinit' can only be applied to a variable with static or thread storage duration" "" { target c++20 } }
+#endif
+}
diff --git a/gcc/testsuite/g++.dg/cpp26/decomp9.C b/gcc/testsuite/g++.dg/cpp26/decomp9.C
index 5629c4c..ee18b60 100644
--- a/gcc/testsuite/g++.dg/cpp26/decomp9.C
+++ b/gcc/testsuite/g++.dg/cpp26/decomp9.C
@@ -63,6 +63,7 @@ foo (const S &&s)
   if (static auto [i, j, k] = t)		// { dg-warning "structured bindings in conditions only available with" "" { target c++23_down } }
     ;						// { dg-error "'static' invalid in condition" "" { target *-*-* } .-1 }
 						// { dg-warning "structured binding declaration can be 'static' only in" "" { target c++17_down } .-2 }
-  if (constexpr auto [i, j, k] = t)		// { dg-warning "structured bindings in conditions only available with" "" { target c++23_down } }
-    ;						// { dg-error "structured binding declaration cannot be 'constexpr'" "" { target *-*-* } .-1 }
+  constexpr T t2 = { 1, 2, 3 };
+  if (constexpr auto [i, j, k] = t2)		// { dg-warning "structured bindings in conditions only available with" "" { target c++23_down } }
+    ;						// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
 }