gdb: don't treat empty enums as flag enums

In C++ it is possible to use an empty enum as a strong typedef.  For
example, a user could write:

  enum class my_type : unsigned char {};

Now my_type can be used like 'unsigned char' except the compiler will
not allow implicit conversion too and from the native 'unsigned char'
type.

This is used in the standard library for things like std::byte.

Currently, when GDB prints a value of type my_type, it looks like
this:

  (gdb) print my_var
  $1 = (unknown: 0x4)

Which isn't great.  This gets worse when we consider something like:

  std::vector<my_type> vec;

When using a pretty-printer, this could look like this:

  std::vector of length 2, capacity 2 = {(unknown: 0x2), (unknown: 0x4)}

Clearly not great.  This is described in PR gdb/30148.

The problem here is in dwarf2/read.c, we assume all enums are flag
enums unless we find an enumerator with a non-flag like value.
Clearly an empty enum contains no non-flag values, so we assume the
enum is a flag enum.

I propose adding an extra check here; that is, an empty enum should
never be a flag enum.

With this the above cases look more like:

  (gdb) print my_var
  $1 = 4

and:

  std::vector of length 2, capacity 2 = {2, 4}

Which look much better.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30148

Reviewed-By: Tom Tromey <tom@tromey.com>

3 files changed, 81 insertions, 0 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 69b2310..038f31c 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -13195,6 +13195,8 @@ update_enumeration_type_from_children (struct die_info *die,
       memcpy (type->fields (), fields.data (),
 	      sizeof (struct field) * fields.size ());
     }
+  else
+    flag_enum = 0;
 
   if (unsigned_enum)
     type->set_is_unsigned (true);
diff --git a/gdb/testsuite/gdb.cp/empty-enum.cc b/gdb/testsuite/gdb.cp/empty-enum.cc
new file mode 100644
index 0000000..1a0eece
--- /dev/null
+++ b/gdb/testsuite/gdb.cp/empty-enum.cc
@@ -0,0 +1,31 @@
+/* Copyright 2023 Free Software Foundation, Inc.
+   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/>.  */
+
+enum enum1 {};
+
+enum class enum2 : unsigned char {};
+
+void
+breakpt (enum1 arg1, enum2 arg2)
+{
+  /* Nothing.  */
+}
+
+int
+main ()
+{
+  breakpt ((enum1) 8, (enum2) 4);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.cp/empty-enum.exp b/gdb/testsuite/gdb.cp/empty-enum.exp
new file mode 100644
index 0000000..83cb8cb
--- /dev/null
+++ b/gdb/testsuite/gdb.cp/empty-enum.exp
@@ -0,0 +1,48 @@
+# Copyright 2023 Free Software Foundation, Inc.
+# 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/>.
+
+# Test how GDB displays empty enums.  At one point an enum with no
+# enumeration values would be considered a flag enum, and, as a
+# consequence any value with that type would display like:
+#
+#   (gdb) print enum_var
+#   $1 = (unknown: 0x8)
+#
+# Which resulted in a lot of noise.  Now GDB treats empty enums as a
+# non-flag enum, and should print them like this:
+#
+#   (gdb) print enum_var
+#   $1 = 8
+#
+# This test checks this behaviour.
+
+standard_testfile .cc
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile]} {
+    return -1
+}
+
+if {![runto_main]} {
+    return -1
+}
+
+gdb_breakpoint "breakpt"
+gdb_continue_to_breakpoint "stop in breakpt"
+
+
+gdb_test "print arg1" " = 8"
+gdb_test "print arg2" " = 4"
+
+gdb_test "ptype arg1" "type = enum enum1 : unsigned int \\{\\}"
+gdb_test "ptype arg2" "type = enum class enum2 : unsigned char \\{\\}"