Index: tools/prettyprinter/Makefile.am
===================================================================
--- tools/prettyprinter/Makefile.am	(revision 5669d0bc8498e99c82f19205874296f37e7fe09e)
+++ tools/prettyprinter/Makefile.am	(revision 510e6f9430710d9525c9d3b03d0d08e6f2fe2de9)
@@ -32,5 +32,5 @@
 nodist_pretty_SOURCES = ${SRC}
 pretty_LDADD = ${LEXLIB} -ldl			# yywrap
-pretty_CXXFLAGS = -Wno-deprecated -Wall -DYY_NO_INPUT -O2 -g -std=c++14
+pretty_CXXFLAGS = -Wno-deprecated -Wall -Wextra -Werror=return-type -DYY_NO_INPUT -O2 -g -std=c++14
 
 MOSTLYCLEANFILES = parser.output