[PATCH][gdb/symtab] Fix element type modification in read_array_type

[Tom de Vries <tdevries@suse.de>]

Hi,

When running test-case gdb.fortran/function-calls.exp with target board
unix/gdb:debug_flags=-gdwarf-5, I run into:
...
(gdb) PASS: gdb.fortran/function-calls.exp: \
  p derived_types_and_module_calls::pass_cart(c)
p derived_types_and_module_calls::pass_cart_nd(c_nd)^M
^M
Program received signal SIGSEGV, Segmentation fault.^M
0x0000000000400f73 in derived_types_and_module_calls::pass_cart_nd \
  (c=<error reading variable: Cannot access memory at address 0xc>) at \
  function-calls.f90:130^M
130             pass_cart_nd = ubound(c%d,1,4)^M
The program being debugged was signaled while in a function called from GDB.^M
GDB has restored the context to what it was before the call.^M
To change this behavior use "set unwindonsignal off".^M
Evaluation of the expression containing the function^M
(derived_types_and_module_calls::pass_cart_nd) will be abandoned.^M
(gdb) FAIL: gdb.fortran/function-calls.exp: p
...

The problem originates in read_array_type, when reading a DW_TAG_array_type
with a dwarf-5 DW_TAG_generic_subrange child.  This is not supported, and the
fallout of this is that rather than constructing a new array type, the code
proceeds to modify the element type.

Fix this conservatively by bailing out in read_array_type when not being able
to construct an array type.

Tested on x86_64-linux.

Any comments?

Thanks,
- Tom

[gdb/symtab] Fix element type modification in read_array_type

gdb/ChangeLog:

2021-02-05  Tom de Vries  <tdevries@suse.de>

	PR symtab/27341
	* dwarf2/read.c (read_array_type): Return NULL when not being able to
	construct an array type.

---
 gdb/dwarf2/read.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index f60e418172c..98189f7b566 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -17243,6 +17243,9 @@ read_array_type (struct die_info *die, struct dwarf2_cu *cu)
 	}
     }
 
+  if (type == element_type)
+    return NULL;
+
   /* Understand Dwarf2 support for vector types (like they occur on
      the PowerPC w/ AltiVec).  Gcc just adds another attribute to the
      array type.  This is not part of the Dwarf2/3 standard yet, but a