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.

5 files changed, 90 insertions, 7 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index c105fb7..82655ea 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,11 @@
 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".
+
+2019-05-29  Tom Tromey  <tromey@adacore.com>
+
 	* inflow.c (struct terminal_info): Add default operator=.
 	* configure: Rebuild.
 	* warning.m4 (AM_GDB_WARNINGS): Add -Wdeprecated-copy,
diff --git a/gdb/cp-valprint.c b/gdb/cp-valprint.c
index ff860df..d8d5c06 100644
--- a/gdb/cp-valprint.c
+++ b/gdb/cp-valprint.c
@@ -329,22 +329,20 @@ cp_print_value_fields (struct type *type, struct type *real_type,
 		}
 	      else if (field_is_static (&TYPE_FIELD (type, i)))
 		{
-		  struct value *v = NULL;
-
 		  try
 		    {
-		      v = value_static_field (type, i);
-		    }
+		      struct value *v = value_static_field (type, i);
 
+		      cp_print_static_field (TYPE_FIELD_TYPE (type, i),
+					     v, stream, recurse + 1,
+					     options);
+		    }
 		  catch (const gdb_exception_error &ex)
 		    {
 		      fprintf_filtered (stream,
 					_("<error reading variable: %s>"),
 					ex.what ());
 		    }
-
-		  cp_print_static_field (TYPE_FIELD_TYPE (type, i),
-					 v, stream, recurse + 1, opts);
 		}
 	      else if (i == vptr_fieldno && type == vptr_basetype)
 		{
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index f795ba0..ddcb834 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,11 @@
 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.
+
+2019-05-29  Tom Tromey  <tromey@adacore.com>
+
 	* gdb.base/finish.exp (finish_no_print): New proc.
 	(finish_tests): Call it.
 
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}"