Fix crash in cp_print_value_fields

PR c++/20020 concerns a crash in cp_print_value_fields.  The immediate
cause is that cp_print_value_fields does not handle the case where
value_static_field fails.  This is fixed in this patch by calling
cp_print_static_field from the "try" block.

Digging a bit deeper, the error occurs because GCC does not emit a
DW_AT_const_value for a static constexpr member appearing in a
template class.  I've filed a GCC bug for this.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-05-29  Tom Tromey  <tromey@adacore.com>

	PR c++/20020:
	* cp-valprint.c (cp_print_value_fields): Call
	cp_print_static_field inside "try".

gdb/testsuite/ChangeLog
2019-05-29  Tom Tromey  <tromey@adacore.com>

	PR c++/20020:
	* gdb.cp/constexpr-field.exp: New file.
	* gdb.cp/constexpr-field.cc: New file.

2 files changed, 73 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.cp/constexpr-field.cc b/gdb/testsuite/gdb.cp/constexpr-field.cc
new file mode 100644
index 0000000..ac2e514
--- /dev/null
+++ b/gdb/testsuite/gdb.cp/constexpr-field.cc
@@ -0,0 +1,37 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2019 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/>.  */
+
+template<typename T>
+struct X
+{
+  static constexpr bool f = true;
+};
+
+X<int> x;
+
+struct Y
+{
+  static constexpr bool f = true;
+};
+
+Y y;
+
+int
+main (void)
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.cp/constexpr-field.exp b/gdb/testsuite/gdb.cp/constexpr-field.exp
new file mode 100644
index 0000000..e4849ef
--- /dev/null
+++ b/gdb/testsuite/gdb.cp/constexpr-field.exp
@@ -0,0 +1,36 @@
+# Copyright 2019 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/>.
+
+# This file is part of the gdb testsuite.
+
+if {[skip_cplus_tests]} { continue }
+
+standard_testfile .cc
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile \
+	 {c++ debug additional_flags=-std=c++17}]} {
+    return -1
+}
+
+if {![runto_main]} {
+    perror "couldn't run to breakpoint"
+    continue
+}
+
+# "x" sometimes isn't available due to
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90526
+gdb_test "print x" " = {static f = .*}"
+
+gdb_test "print y" " = {static f = true}"