From: Jakub Jelinek <jakub@redhat.com>
Date: Wed, 8 Apr 2026 05:46:38 +0000 (+0200)
Subject: c++: Implement CWG3135 - constexpr structured bindings with prvalues from tuples
X-Git-Tag: basepoints/gcc-17~284
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=7e4e9dd9eb01de7bf9fc7abe82e9ab01123283cc;p=thirdparty%2Fgcc.git

c++: Implement CWG3135 - constexpr structured bindings with prvalues from tuples

The following patch implements another CWG issue, added e.g. for the
P1789R3 (Library Support for Expansion Statements) APIs where get returns
a prvalue rather than a reference.

While the issue has been voted as a non-DR, this implementation assumes it
will be changed to a DR.  If that won't be the case, we will need to limit
the changes to C++26 and later only.

The gomp testcase was using get returning prvalue and the changes
cause some changes in lines on which stuff is reported.

2026-04-08  Jakub Jelinek  <jakub@redhat.com>

	* decl.cc (cp_finish_decomp): Implement CWG3135 - constexpr structured
	bindings with prvalues from tuples (as a DR).  Don't call
	cp_build_reference_type if init is prvalue.

	* g++.dg/cpp26/decomp30.C: New test.
	* g++.dg/gomp/pr108503.C: Adjust expected diagnostic locations.

Reviewed-by: Jason Merrill <jason@redhat.com>
---

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 930253d9646..ac74f4930e2 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -11025,7 +11025,8 @@ cp_finish_decomp (tree decl, cp_decomp *decomp, bool test_p)
 		  maybe_push_decl (t);
 		  /* Save the decltype away before reference collapse.  */
 		  hash_map_safe_put<hm_ggc> (decomp_type_table, t, eltype);
-		  eltype = cp_build_reference_type (eltype, !lvalue_p (init));
+		  if (glvalue_p (init))
+		    eltype = cp_build_reference_type (eltype, !lvalue_p (init));
 		  TREE_TYPE (t) = eltype;
 		  layout_decl (t, 0);
 		  DECL_HAS_VALUE_EXPR_P (t) = 0;
@@ -11070,7 +11071,8 @@ cp_finish_decomp (tree decl, cp_decomp *decomp, bool test_p)
 	    }
 	  /* Save the decltype away before reference collapse.  */
 	  hash_map_safe_put<hm_ggc> (decomp_type_table, v[i], eltype);
-	  eltype = cp_build_reference_type (eltype, !lvalue_p (init));
+	  if (glvalue_p (init))
+	    eltype = cp_build_reference_type (eltype, !lvalue_p (init));
 	  TREE_TYPE (v[i]) = eltype;
 	  layout_decl (v[i], 0);
 	  if (DECL_HAS_VALUE_EXPR_P (v[i]))
diff --git a/gcc/testsuite/g++.dg/cpp26/decomp30.C b/gcc/testsuite/g++.dg/cpp26/decomp30.C
new file mode 100644
index 00000000000..f1e100e0f2b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/decomp30.C
@@ -0,0 +1,35 @@
+// CWG3135 - constexpr structured bindings with prvalues from tuples
+// { dg-do compile { target c++14 } }
+// { dg-options "" }
+
+namespace std {
+  template <typename T> struct tuple_size;
+  template <int, typename> struct tuple_element;
+}
+
+struct A {
+  template <int I> constexpr int get () const { return I + 6; }
+};
+
+template <> struct std::tuple_size <A> { static const int value = 2; };
+template <int I> struct std::tuple_element <I, A> { using type = int; };
+template <> struct std::tuple_size <const A> { static const int value = 2; };
+template <int I> struct std::tuple_element <I, const A> { using type = const int; };
+
+constexpr A
+bar ()
+{
+  return A {};
+}
+
+template <int N>
+constexpr int
+foo ()
+{
+  constexpr auto [...i] = bar ();	// { dg-warning "structured binding packs only available with" "" { target { c++17 && c++23_down } } }
+					// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+					// { dg-warning "structured bindings only available with" "" { target c++14_down } .-2 }
+  return i...[0] + i...[1];		// { dg-warning "pack indexing only available with" "" { target c++23_down } }
+}
+
+static_assert (foo <42> () == 13);
diff --git a/gcc/testsuite/g++.dg/gomp/pr108503.C b/gcc/testsuite/g++.dg/gomp/pr108503.C
index 906d41b9a8f..5c6c08ba0cb 100644
--- a/gcc/testsuite/g++.dg/gomp/pr108503.C
+++ b/gcc/testsuite/g++.dg/gomp/pr108503.C
@@ -21,7 +21,7 @@ void
 foo (B a)
 {
   #pragma omp for collapse(2)
-  for (auto [i, j, k] : a)
-    for (int l = i; l < j; l += k)	// { dg-error "initializer expression refers to iteration variable 'i'" }
-      ;					// { dg-error "condition expression refers to iteration variable 'j'" "" { target *-*-* } .-1 }
-}					// { dg-error "increment expression refers to iteration variable 'k'" "" { target *-*-* } .-2 }
+  for (auto [i, j, k] : a)		// { dg-error "initializer expression refers to iteration variable 'i'" }
+    for (int l = i; l < j; l += k)	// { dg-error "condition expression refers to iteration variable 'j'" }
+      ;					// { dg-error "increment expression refers to iteration variable 'k'" "" { target *-*-* } .-2 }
+}