Allow a pretty-printer without a to_string method

PR python/16047 points out that, while the documentation says that the
to_string method is optional for a pretty-printer, the code disagrees
and throws an exception.  This patch fixes the problem.  varobj is
already ok here.

Tested on x86-64 Fedora 26.

gdb/ChangeLog
2018-09-08  Tom Tromey  <tom@tromey.com>

	PR python/16047:
	* python/py-prettyprint.c (pretty_print_one_value): Check for
	to_string method.

gdb/testsuite/ChangeLog
2018-09-08  Tom Tromey  <tom@tromey.com>

	PR python/16047:
	* gdb.python/py-prettyprint.py (pp_int_typedef3): New class.
	(register_pretty_printers): Register new printer.
	* gdb.python/py-prettyprint.exp (run_lang_tests): Add int_type3
	test.
	* gdb.python/py-prettyprint.c (int_type3): New typedef.
	(an_int_type3): New global.

1 files changed, 10 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.python/py-prettyprint.py b/gdb/testsuite/gdb.python/py-prettyprint.py
index 7357f05cc93..91592b4ec79 100644
--- a/gdb/testsuite/gdb.python/py-prettyprint.py
+++ b/gdb/testsuite/gdb.python/py-prettyprint.py
@@ -253,6 +253,15 @@ class pp_int_typedef (object):
     def to_string(self):
         return "type=%s, val=%s" % (self.val.type, int(self.val))
 
+class pp_int_typedef3 (object):
+    "A printer without a to_string method"
+
+    def __init__(self, val):
+        self.val = val
+
+    def children(self):
+        yield 's', 27
+
 def lookup_function (val):
     "Look-up and return a pretty-printer that can print val."
 
@@ -362,6 +371,7 @@ def register_pretty_printers ():
 
     typedefs_pretty_printers_dict[re.compile ('^int_type$')] = pp_int_typedef
     typedefs_pretty_printers_dict[re.compile ('^int_type2$')] = pp_int_typedef
+    typedefs_pretty_printers_dict[re.compile ('^int_type3$')] = pp_int_typedef3
 
 # Dict for struct types with typedefs fully stripped.
 pretty_printers_dict = {}