Don't compare types of enum fields

Comparing types of enum fields results in a crash, because they don't
have a type.

It can be reproduced by comparing the types of 2 instances of the same
enum type in different objects:

enum.h:

    enum e
    {
      zero,
      one,
    };

enum-1.c:

    #include <enum.h>
    int func();
    enum e e1;
    int main()
    {
      return e1 + func();
    }

enum-2.c:

    #include <enum.h>
    enum e e2;
    int func()
    {
      return e2;
    }

$ gcc -g -oenum enum-1.c enum-2.c
$ gdb -q enum.exe
Reading symbols from enum.exe...
(gdb) py print(gdb.parse_and_eval("e1").type==gdb.parse_and_eval("e2").type)

Thread 1 received signal SIGSEGV, Segmentation fault.
[Switching to Thread 6184.0x1cc4]
check_typedef (type=0x0) at C:/src/repos/binutils-gdb.git/gdb/gdbtypes.c:2745
2745      while (type->code () == TYPE_CODE_TYPEDEF)

gdb/ChangeLog:

2020-12-19  Hannes Domani  <ssbssa@yahoo.de>

	PR exp/27070
	* gdbtypes.c (check_types_equal): Don't compare types of enum fields.

gdb/testsuite/ChangeLog:

2020-12-19  Hannes Domani  <ssbssa@yahoo.de>

	PR exp/27070
	* gdb.python/compare-enum-type-a.c: New test.
	* gdb.python/compare-enum-type-b.c: New test.
	* gdb.python/compare-enum-type.exp: New file.
	* gdb.python/compare-enum-type.h: New test.

gdb/testsuite/gdb.python/compare-enum-type-a.c

@@ -0,0 +1,28 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 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/>.  */
+
+#include "compare-enum-type.h"
+
+int func (void);
+
+enum e e1;
+
+int
+main (void)
+{
+  return e1 + func ();
+}

gdb/testsuite/gdb.python/compare-enum-type-b.c

@@ -0,0 +1,26 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 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/>.  */
+
+#include "compare-enum-type.h"
+
+enum e e2;
+
+int
+func (void)
+{
+  return e2;
+}

gdb/testsuite/gdb.python/compare-enum-type.exp

@@ -0,0 +1,27 @@
+# Copyright 2020 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 if enum type comparison succeeds (PR exp/27070).
+
+standard_testfile -a.c -b.c
+
+if { [prepare_for_testing "failed to prepare" "compare-enum-type" \
+	  [list $srcfile $srcfile2]] } {
+    return
+}
+
+if { [skip_python_tests] } { continue }
+
+gdb_test "py print(gdb.parse_and_eval('e1').type == gdb.parse_and_eval('e2').type)" "True"

gdb/testsuite/gdb.python/compare-enum-type.h

@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 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 e
+{
+  zero,
+  one,
+};