[gcc r16-9509] c++: Fix structured binding pack instantiation ICE [PR125591]

https://gcc.gnu.org/g:c40ebe81cd414dd691a065fb4273d0e3111b5225

commit r16-9509-gc40ebe81cd414dd691a065fb4273d0e3111b5225
Author: Jakub Jelinek <[email protected]>
Date:   Thu Aug 6 11:13:53 2026 +0200

    c++: Fix structured binding pack instantiation ICE [PR125591]
    
    The following testcase ICEs in tsubst_pack_expansion, we trigger the
    gcc_assert (DECL_DECOMPOSITION_P (orig_arg));
    assertion.  But in this case, retrieve_local_specialization doesn't
    return the expected DECL_DECOMPOSITION_P, but ARGUMENT_PACK_SELECT instead.
    That is because it has been registered earlier in
    gen_elem_of_pack_expansion_instantiation
    in
                  aps = make_argument_pack_select (arg_pack, index);
                  if (!mark_used (parm, complain) && !(complain & tf_error))
                    return error_mark_node;
                  register_local_specialization (aps, parm);
    The following patch just stops assuming retrieve_local_specialization
    has to return DECL_DECOMPOSITION_P, but allows also ARGUMENT_PACK_SELECT.
    The patch is large due to reindentation, with diff -upb it is just
    
    @@ -14309,8 +14309,9 @@ tsubst_pack_expansion (tree t, tree args
           else if (DECL_DECOMPOSITION_P (parm_pack))
            {
              orig_arg = retrieve_local_specialization (parm_pack);
    +         if (DECL_DECOMPOSITION_P (orig_arg))
    +           {
            expand_sb_pack:
    -         gcc_assert (DECL_DECOMPOSITION_P (orig_arg));
              if (TREE_TYPE (orig_arg) == error_mark_node)
                return error_mark_node;
              gcc_assert (DECL_HAS_VALUE_EXPR_P (orig_arg));
    @@ -14339,6 +14340,12 @@ tsubst_pack_expansion (tree t, tree args
                }
            }
           else
    +           {
    +             gcc_assert (TREE_CODE (orig_arg) == ARGUMENT_PACK_SELECT);
    +             arg_pack = orig_arg;
    +           }
    +       }
    +      else
             {
              int idx;
               template_parm_level_and_index (parm_pack, &level, &idx);
    
    2026-08-06  Jakub Jelinek  <[email protected]>
    
            PR c++/125591
            * pt.cc (tsubst_pack_expansion): Don't require
            retrieve_local_specialization on structured binding to
            always return structured binding, instead handle if it returns
            ARGUMENT_PACK_SELECT.
    
            * g++.dg/cpp26/decomp31.C: New test.
    
    Reviewed-by: Jason Merrill <[email protected]>
    (cherry picked from commit 091a150a8a51bc953906d3da2a1a09d83fc36633)

Diff:
---
 gcc/cp/pt.cc                          | 53 ++++++++++++++++++++---------------
 gcc/testsuite/g++.dg/cpp26/decomp31.C | 19 +++++++++++++
 2 files changed, 49 insertions(+), 23 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 4359ae8e5569..729c7b191654 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -14254,33 +14254,40 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
       else if (DECL_DECOMPOSITION_P (parm_pack))
 	{
 	  orig_arg = retrieve_local_specialization (parm_pack);
-	expand_sb_pack:
-	  gcc_assert (DECL_DECOMPOSITION_P (orig_arg));
-	  if (TREE_TYPE (orig_arg) == error_mark_node)
-	    return error_mark_node;
-	  gcc_assert (DECL_HAS_VALUE_EXPR_P (orig_arg));
-	  arg_pack = DECL_VALUE_EXPR (orig_arg);
-	  if (TREE_CODE (arg_pack) != ARRAY_REF)
+	  if (DECL_DECOMPOSITION_P (orig_arg))
 	    {
-	      /* Structured binding packs when initializer is non-dependent
-		 should have their DECL_VALUE_EXPR set to a TREE_VEC.  See
-		 cp_finish_decomp comment above the packv variable for
-		 details.  */
-	      tree vec = make_tree_vec (TREE_VEC_LENGTH (arg_pack) - 2);
-	      if (TREE_VEC_LENGTH (vec))
-		memcpy (TREE_VEC_BEGIN (vec), &TREE_VEC_ELT (arg_pack, 2),
-			TREE_VEC_LENGTH (vec) * sizeof (tree));
-	      arg_pack = make_node (NONTYPE_ARGUMENT_PACK);
-	      ARGUMENT_PACK_ARGS (arg_pack) = vec;
+	    expand_sb_pack:
+	      if (TREE_TYPE (orig_arg) == error_mark_node)
+		return error_mark_node;
+	      gcc_assert (DECL_HAS_VALUE_EXPR_P (orig_arg));
+	      arg_pack = DECL_VALUE_EXPR (orig_arg);
+	      if (TREE_CODE (arg_pack) != ARRAY_REF)
+		{
+		  /* Structured binding packs when initializer is non-dependent
+		     should have their DECL_VALUE_EXPR set to a TREE_VEC.  See
+		     cp_finish_decomp comment above the packv variable for
+		     details.  */
+		  tree vec = make_tree_vec (TREE_VEC_LENGTH (arg_pack) - 2);
+		  if (TREE_VEC_LENGTH (vec))
+		    memcpy (TREE_VEC_BEGIN (vec), &TREE_VEC_ELT (arg_pack, 2),
+			    TREE_VEC_LENGTH (vec) * sizeof (tree));
+		  arg_pack = make_node (NONTYPE_ARGUMENT_PACK);
+		  ARGUMENT_PACK_ARGS (arg_pack) = vec;
+		}
+	      else
+		{
+		  /* If the structured binding pack has type dependent
+		     base, we can't expand it yet.  */
+		  tree base = TREE_OPERAND (arg_pack, 0);
+		  gcc_assert (VAR_P (base)
+			      && type_dependent_expression_p (base));
+		  arg_pack = NULL_TREE;
+		}
 	    }
 	  else
 	    {
-	      /* If the structured binding pack has type dependent
-		 base, we can't expand it yet.  */
-	      tree base = TREE_OPERAND (arg_pack, 0);
-	      gcc_assert (VAR_P (base)
-			  && type_dependent_expression_p (base));
-	      arg_pack = NULL_TREE;
+	      gcc_assert (TREE_CODE (orig_arg) == ARGUMENT_PACK_SELECT);
+	      arg_pack = orig_arg;
 	    }
 	}
       else
diff --git a/gcc/testsuite/g++.dg/cpp26/decomp31.C b/gcc/testsuite/g++.dg/cpp26/decomp31.C
new file mode 100644
index 000000000000..f9657a9ca6cb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/decomp31.C
@@ -0,0 +1,19 @@
+// PR c++/125591
+// { dg-do compile { target c++20 } }
+// { dg-options "" }
+
+template <typename T, typename U>
+concept is_same_v = __is_same (T, U);
+
+struct A { int x, y, z; };
+struct B { int x; long y; };
+
+template <class V>
+consteval bool
+foo ()
+{
+  constexpr auto [...Ms] = V {};		// { dg-warning "structured binding packs only available with" "" { target c++23_down } }
+						// { dg-warning "structured binding declaration can be 'constexpr' only with" "" { target c++23_down } .-1 }
+  using T = decltype (Ms...[0]);		// { dg-warning "pack indexing only available with" "" { target c++23_down } }
+  return (is_same_v<decltype(Ms), T> && ...);
+}