Fix .debug_names regression with new indexer

At AdaCore, we run the internal gdb test suite in several modes,
including one using the .debug_names index.  This caught a regression
caused by the new DWARF indexer.

First, the psymtabs-based .debug_names generator was completely wrong.
However, to avoid making the rewrite series even bigger (fixing the
writer will also require rewriting the .debug_names reader), it
attempted to preserve the weirdness.

However, this was not done properly.  For example the old writer did
this:

-      case STRUCT_DOMAIN:
-	return DW_TAG_structure_type;

The new code, instead, simply preserves the actual DWARF tag -- but
this makes future lookups fail, because the .debug_names reader only
looks for DW_TAG_structure_type.

This patch attempts to revert to the old behavior in the writer.

gdb/dwarf2/cooked-index.h

@@ -32,6 +32,7 @@
 #include "gdbsupport/iterator-range.h"
 #include "gdbsupport/thread-pool.h"
 #include "dwarf2/mapped-index.h"
+#include "dwarf2/tag.h"
 
 struct dwarf2_per_cu_data;
 
@@ -107,11 +108,13 @@ struct cooked_index_entry : public allocate_on_obstack
     switch (kind)
       {
       case VARIABLES_DOMAIN:
-	return tag == DW_TAG_variable;
+	return (tag == DW_TAG_variable
+		|| tag == DW_TAG_constant
+		|| tag == DW_TAG_enumerator);
       case FUNCTIONS_DOMAIN:
 	return tag == DW_TAG_subprogram;
       case TYPES_DOMAIN:
-	return tag == DW_TAG_typedef || tag == DW_TAG_structure_type;
+	return tag_is_type (tag);
       case MODULES_DOMAIN:
 	return tag == DW_TAG_module;
       }

gdb/dwarf2/index-write.c

@@ -37,6 +37,7 @@
 #include "gdbcmd.h"
 #include "objfiles.h"
 #include "ada-lang.h"
+#include "dwarf2/tag.h"
 
 #include <algorithm>
 #include <cmath>
@@ -569,7 +570,18 @@ public:
     const auto it = m_cu_index_htab.find (entry->per_cu);
     gdb_assert (it != m_cu_index_htab.cend ());
     const char *name = entry->full_name (&m_string_obstack);
-    insert (entry->tag, name, it->second, (entry->flags & IS_STATIC) != 0,
+
+    /* This is incorrect but it mirrors gdb's historical behavior; and
+       because the current .debug_names generation is also incorrect,
+       it seems better to follow what was done before, rather than
+       introduce a mismatch between the newer and older gdb.  */
+    dwarf_tag tag = entry->tag;
+    if (tag != DW_TAG_typedef && tag_is_type (tag))
+      tag = DW_TAG_structure_type;
+    else if (tag == DW_TAG_enumerator || tag == DW_TAG_constant)
+      tag = DW_TAG_variable;
+
+    insert (tag, name, it->second, (entry->flags & IS_STATIC) != 0,
 	    entry->per_cu->is_debug_types ? unit_kind::tu : unit_kind::cu,
 	    entry->per_cu->lang);
   }

gdb/dwarf2/tag.h

@@ -0,0 +1,67 @@
+/* Tag attributes
+
+   Copyright (C) 2022 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_DWARF2_TAG_H
+#define GDB_DWARF2_TAG_H
+
+#include "dwarf2.h"
+
+/* Return true if TAG represents a type, false otherwise.  */
+
+static inline bool
+tag_is_type (dwarf_tag tag)
+{
+  switch (tag)
+    {
+    case DW_TAG_padding:
+    case DW_TAG_array_type:
+    case DW_TAG_class_type:
+    case DW_TAG_enumeration_type:
+    case DW_TAG_pointer_type:
+    case DW_TAG_reference_type:
+    case DW_TAG_string_type:
+    case DW_TAG_structure_type:
+    case DW_TAG_subroutine_type:
+    case DW_TAG_typedef:
+    case DW_TAG_union_type:
+    case DW_TAG_ptr_to_member_type:
+    case DW_TAG_set_type:
+    case DW_TAG_subrange_type:
+    case DW_TAG_base_type:
+    case DW_TAG_const_type:
+    case DW_TAG_packed_type:
+    case DW_TAG_template_type_param:
+    case DW_TAG_volatile_type:
+    case DW_TAG_restrict_type:
+    case DW_TAG_interface_type:
+    case DW_TAG_namespace:
+    case DW_TAG_unspecified_type:
+    case DW_TAG_shared_type:
+    case DW_TAG_rvalue_reference_type:
+    case DW_TAG_coarray_type:
+    case DW_TAG_dynamic_type:
+    case DW_TAG_atomic_type:
+    case DW_TAG_immutable_type:
+      return true;
+    default:
+      return false;
+    }
+}
+
+#endif /* GDB_DWARF2_TAG_H */