c++: Fix up decltype of non-dependent structured binding decl in template [PR92687]

finish_decltype_type uses DECL_HAS_VALUE_EXPR_P (expr) check for
DECL_DECOMPOSITION_P (expr) to determine if it is
array/struct/vector/complex etc. subobject proxy case vs. structured
binding using std::tuple_{size,element}.
For non-templates or when templates are already instantiated, that works
correctly, finalized DECL_DECOMPOSITION_P non-base vars indeed have
DECL_VALUE_EXPR in the former case and don't have it in the latter.
It works fine for dependent structured bindings as well, cp_finish_decomp in
that case creates DECLTYPE_TYPE tree and defers the handling until
instantiation.
As the testcase shows, this doesn't work for the non-dependent structured
binding case in templates, because DECL_HAS_VALUE_EXPR_P is set in that case
always; cp_finish_decomp ends with:
  if (processing_template_decl)
    {
      for (unsigned int i = 0; i < count; i++)
        if (!DECL_HAS_VALUE_EXPR_P (v[i]))
          {
            tree a = build_nt (ARRAY_REF, decl, size_int (i),
                               NULL_TREE, NULL_TREE);
            SET_DECL_VALUE_EXPR (v[i], a);
            DECL_HAS_VALUE_EXPR_P (v[i]) = 1;
          }
    }
and those artificial ARRAY_REFs are used in various places during
instantiation to find out what base the DECL_DECOMPOSITION_P VAR_DECLs
have and their positions.

The following patch fixes that by changing lookup_decomp_type, such that
it doesn't ICE when called on a DECL_DECOMPOSITION_P var which isn't in a
hash table, but returns NULL_TREE in that case, and for processing_template_decl
asserts DECL_HAS_VALUE_EXPR_P is non-NULL and just calls lookup_decomp_type.
If it returns non-NULL, it is a structured binding using tuple and its result
is returned, otherwise it falls through to returning unlowered_expr_type (expr)
because it is an array, structure etc. subobject proxy.
For !processing_template_decl it keeps doing what it did before,
DECL_HAS_VALUE_EXPR_P meaning it is an array/structure etc. subobject proxy,
otherwise the tuple case.

2024-03-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/92687
	* decl.cc (lookup_decomp_type): Return NULL_TREE if decomp_type_table
	doesn't have entry for V.
	* semantics.cc (finish_decltype_type): If ptds.saved, assert
	DECL_HAS_VALUE_EXPR_P is true and decide on tuple vs. non-tuple based
	on if lookup_decomp_type is NULL or not.

	* g++.dg/cpp1z/decomp59.C: New test.

3 files changed, 75 insertions, 1 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 05e4600c7bb..993d7ef4d2b 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -9262,7 +9262,9 @@ static GTY((cache)) decl_tree_cache_map *decomp_type_table;
 tree
 lookup_decomp_type (tree v)
 {
-  return *decomp_type_table->get (v);
+  if (tree *slot = decomp_type_table->get (v))
+    return *slot;
+  return NULL_TREE;
 }
 
 /* Mangle a decomposition declaration if needed.  Arguments like
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 57840176863..adb1ba48d29 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -11804,6 +11804,15 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
 	 access expression).  */
       if (DECL_DECOMPOSITION_P (expr))
 	{
+	  if (ptds.saved)
+	    {
+	      gcc_checking_assert (DECL_HAS_VALUE_EXPR_P (expr));
+	      /* DECL_HAS_VALUE_EXPR_P is always set if
+		 processing_template_decl.  If lookup_decomp_type
+		 returns non-NULL, it is the tuple case.  */
+	      if (tree ret = lookup_decomp_type (expr))
+		return ret;
+	    }
 	  if (DECL_HAS_VALUE_EXPR_P (expr))
 	    /* Expr is an array or struct subobject proxy, handle
 	       bit-fields properly.  */
diff --git a/gcc/testsuite/g++.dg/cpp1z/decomp59.C b/gcc/testsuite/g++.dg/cpp1z/decomp59.C
new file mode 100644
index 00000000000..52a72fa9c08
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/decomp59.C
@@ -0,0 +1,63 @@
+// PR c++/92687
+// { 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;
+  template <int I> int& get() { return i; }
+};
+
+template<> struct std::tuple_size<A> { static const int value = 2; };
+template<int I> struct std::tuple_element<I,A> { using type = int; };
+
+template<typename T>
+struct is_reference {
+  static const bool value = false;
+};
+
+template<typename T>
+struct is_reference<T&>
+{
+  static const bool value = true;
+};
+
+template<typename T>
+struct is_reference<T&&>
+{
+  static const bool value = true;
+};
+
+template<int N>
+void
+foo ()
+{
+  auto [x, y] = A {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+  static_assert (!is_reference<decltype (x)>::value, "");
+}
+
+void
+bar ()
+{
+  auto [x, y] = A {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+  static_assert (!is_reference<decltype (x)>::value, "");
+}
+
+template<typename T>
+void
+baz ()
+{
+  auto [x, y] = T {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+  static_assert (!is_reference<decltype (x)>::value, "");
+}
+
+void
+qux ()
+{
+  foo<0> ();
+  baz<A> ();
+}