[PATCH] dwarf2: avoid decoding .debug_line for type units

[Bratislav Filipovic <bfilipov@amd.com>]

Running gdb.dwarf2/pr13961.exp with clang fails with an internal error:

  gdb/dwarf2/read.c: internal-error: decode_line_header_for_cu:
  Assertion `!cu->per_cu->is_debug_types ()' failed.

pr13961 is a legacy .S test that references a .debug_line label from
both a CU and a TU.  The assembly includes an empty .debug_line section
declaration:

  .section .debug_line,"",%progbits
.Ldebug_line0:

With gcc this results in a dummy (valid, but content-less) .debug_line
header being emitted, so GDB can build a line header and resolve file
indices.  With clang the .debug_line section can be missing/empty,
leaving the line header unset.

During symbol creation, new_symbol may then try to lazily decode the
CU-only line header while processing a type unit, which triggers the
assertion above.

Fix this by only decoding the line header for non-type units.  If no
line header is available, emit a complaint and continue without setting
the symtab rather than attempting CU-only line decoding from a TU.

Tested: gdb.dwarf2/pr13961.exp (CC_FOR_TARGET=clang-23)
---
 gdb/dwarf2/read.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index bc7b8b46d87..655cc753334 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -15430,21 +15430,21 @@ is_ada_import_or_export (dwarf2_cu *cu, const char *name,
 
 static void
 new_symbol_file_line (struct die_info *die, struct dwarf2_cu *cu,
-		      struct symbol *sym)
+                      struct symbol *sym)
 {
   bool inlined_func = (die->tag == DW_TAG_inlined_subroutine);
 
   /* Handle DW_AT_call_line / DW_AT_decl_line.  */
   struct attribute *attr
     = dwarf2_attr (die, inlined_func ? DW_AT_call_line : DW_AT_decl_line,
-		   cu);
+                   cu);
   if (attr != nullptr)
     sym->set_line (attr->unsigned_constant ().value_or (0));
 
   /* Handle DW_AT_call_file / DW_AT_decl_file.  */
   struct dwarf2_cu *file_cu = cu;
   attr = dwarf2_attr (die, inlined_func ? DW_AT_call_file : DW_AT_decl_file,
-		      &file_cu);
+                      &file_cu);
   if (attr == nullptr)
     return;
 
@@ -15452,16 +15452,27 @@ new_symbol_file_line (struct die_info *die, struct dwarf2_cu *cu,
   if (!index_cst.has_value ())
     return;
 
-  if (file_cu->line_header == nullptr)
+  /* decode_line_header_for_cu is CU-only and asserts on debug_types.
+     For type units, the TU setup code runs before processing child DIEs,
+     so if line_header is still nullptr here it means there is no usable
+     line table for this unit.  */
+  if (file_cu->line_header == nullptr
+      && !file_cu->per_cu->is_debug_types ())
     {
       file_and_directory fnd (nullptr, nullptr);
       decode_line_header_for_cu (file_cu->dies, file_cu, fnd);
     }
 
   file_name_index file_index = (file_name_index) *index_cst;
-  struct file_entry *fe = nullptr;
-  if (file_cu->line_header != nullptr)
-    fe = file_cu->line_header->file_name_at (file_index);
+
+  /* Check if we successfully got line_header.  */
+  if (file_cu->line_header == nullptr)
+    {
+      complaint (_("missing .debug_line information to resolve file index"));
+      return;
+    }
+
+  struct file_entry *fe = file_cu->line_header->file_name_at (file_index);
 
   if (fe == nullptr)
     {
-- 
2.43.0