[Bug exp/26875] New: Incorrect value printed for address of first element of zero-length array

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26875

            Bug ID: 26875
           Summary: Incorrect value printed for address of first element
                    of zero-length array
           Product: gdb
           Version: unknown
            Status: NEW
          Severity: normal
          Priority: P2
         Component: exp
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Consider test-case gdb.base/arrayidx.exp, augmented with fedora patch
https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-archer-vla-tests.patch:
...
diff --git a/gdb/testsuite/gdb.base/arrayidx.c
b/gdb/testsuite/gdb.base/arrayidx.c
--- a/gdb/testsuite/gdb.base/arrayidx.c
+++ b/gdb/testsuite/gdb.base/arrayidx.c
@@ -17,6 +17,13 @@

 int array[] = {1, 2, 3, 4};

+#ifdef __GNUC__
+struct
+  {
+    int a[0];
+  } unbound;
+#endif
+
 int
 main (void)
 {
diff --git a/gdb/testsuite/gdb.base/arrayidx.exp
b/gdb/testsuite/gdb.base/arrayidx.exp
--- a/gdb/testsuite/gdb.base/arrayidx.exp
+++ b/gdb/testsuite/gdb.base/arrayidx.exp
@@ -49,4 +49,12 @@ gdb_test "print array" \
          "\\{\\\[0\\\] = 1, \\\[1\\\] = 2, \\\[2\\\] = 3, \\\[3\\\] = 4\\}" \
          "print array with array-indexes on"

-
+set test "p unbound.a == &unbound.a\[0\]"
+gdb_test_multiple $test $test {
+    -re " = 1\r\n$gdb_prompt $" {
+       pass $test
+    }
+    -re "No symbol \"unbound\" in current context.\r\n$gdb_prompt $" {
+       unsupported "$test (no GCC)"
+    }
+}
...

We run into this FAIL:
...
p unbound.a == &unbound.a[0]^M
$7 = 0^M
(gdb) FAIL: gdb.base/arrayidx.exp: p unbound.a == &unbound.a[0]
...

A bit more probing:
...
(gdb) p unbound
$3 = {a = 0x601044}
(gdb) p &unbound
$4 = (struct {...} *) 0x601044
(gdb) p unbound.a
$5 = 0x601044
(gdb) p &unbound.a[0]
$6 = (int *) 0xffffffffefc93d84^M
...
reveals that we have a problem printing the address of the first element of a
zero length array.

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