Index: driver/Makefile.am
===================================================================
--- driver/Makefile.am	(revision a554e5f423752296e46ab9ced5a34d32676e55d5)
+++ driver/Makefile.am	(revision 850aff17345d3e7433f854ef3f1b65369f8016d8)
@@ -19,5 +19,5 @@
 
 # applies to both programs
-AM_CXXFLAGS = @HOST_FLAGS@ -Wall -O2 -g -std=c++14 -I${abs_top_srcdir}/src -I${abs_top_srcdir}/src/include
+AM_CXXFLAGS = @HOST_FLAGS@ -Wall -Wextra -Werror=return-type -O2 -g -std=c++14 -I${abs_top_srcdir}/src -I${abs_top_srcdir}/src/include
 
 # don't install cfa directly
Index: libcfa/prelude/Makefile.am
===================================================================
--- libcfa/prelude/Makefile.am	(revision a554e5f423752296e46ab9ced5a34d32676e55d5)
+++ libcfa/prelude/Makefile.am	(revision 850aff17345d3e7433f854ef3f1b65369f8016d8)
@@ -26,5 +26,5 @@
 
 CC = @LOCAL_CFACC@
-AM_CFLAGS = -g -Wall -Wno-unused-function -fPIC @ARCH_FLAGS@ @CONFIG_CFLAGS@
+AM_CFLAGS = -g -Wall -Werror=return-type -Wno-unused-function -fPIC @ARCH_FLAGS@ @CONFIG_CFLAGS@
 AM_CFAFLAGS = @CONFIG_CFAFLAGS@
 
Index: libcfa/src/Makefile.am
===================================================================
--- libcfa/src/Makefile.am	(revision a554e5f423752296e46ab9ced5a34d32676e55d5)
+++ libcfa/src/Makefile.am	(revision 850aff17345d3e7433f854ef3f1b65369f8016d8)
@@ -33,6 +33,6 @@
 # The built sources must not depend on the installed inst_headers_src
 AM_CFAFLAGS = -quiet -cfalib -I$(srcdir)/stdhdr -I$(srcdir)/concurrency $(if $(findstring ${gdbwaittarget}, ${@}), -XCFA --gdb) @CONFIG_CFAFLAGS@
-AM_CFLAGS = -g -Wall -Wno-unused-function -fPIC -fexceptions -pthread @ARCH_FLAGS@ @CONFIG_CFLAGS@
-AM_CCASFLAGS = -g -Wall -Wno-unused-function @ARCH_FLAGS@ @CONFIG_CFLAGS@
+AM_CFLAGS = -g -Wall -Werror=return-type -Wno-unused-function -fPIC -fexceptions -pthread @ARCH_FLAGS@ @CONFIG_CFLAGS@
+AM_CCASFLAGS = -g -Wall -Werror=return-type -Wno-unused-function @ARCH_FLAGS@ @CONFIG_CFLAGS@
 CFACC = @CFACC@
 
Index: longrun_tests/Makefile.am
===================================================================
--- longrun_tests/Makefile.am	(revision a554e5f423752296e46ab9ced5a34d32676e55d5)
+++ longrun_tests/Makefile.am	(revision 850aff17345d3e7433f854ef3f1b65369f8016d8)
@@ -36,4 +36,6 @@
 	-g \
 	-Wall \
+	-Wextra \
+	-Werror=return-type
 	-Wno-unused-function \
 	-quiet \
Index: src/Makefile.am
===================================================================
--- src/Makefile.am	(revision a554e5f423752296e46ab9ced5a34d32676e55d5)
+++ src/Makefile.am	(revision 850aff17345d3e7433f854ef3f1b65369f8016d8)
@@ -59,5 +59,5 @@
 
 $(srcdir)/AST/Type.hpp : BasicTypes-gen.cc
-	${AM_V_GEN}${CXXCOMPILE} $< -o BasicTypes-gen -Wall -Wextra
+	${AM_V_GEN}${CXXCOMPILE} $< -o BasicTypes-gen -Wall -Wextra -Werror=return-type
 	@./BasicTypes-gen
 	@rm BasicTypes-gen
@@ -71,5 +71,5 @@
 EXTRA_DIST = include/cassert include/optional BasicTypes-gen.cc
 
-AM_CXXFLAGS = @HOST_FLAGS@ -Wno-deprecated -Wall -Wextra -DDEBUG_ALL -I./Parser -I$(srcdir)/Parser -I$(srcdir)/include -DYY_NO_INPUT -O3 -g -std=c++14 $(TCMALLOCFLAG)
+AM_CXXFLAGS = @HOST_FLAGS@ -Wno-deprecated -Wall -Wextra -Werror=return-type -DDEBUG_ALL -I./Parser -I$(srcdir)/Parser -I$(srcdir)/include -DYY_NO_INPUT -O3 -g -std=c++14 $(TCMALLOCFLAG)
 AM_LDFLAGS  = @HOST_FLAGS@ -Xlinker -export-dynamic
 ARFLAGS     = cr
Index: tests/Makefile.am
===================================================================
--- tests/Makefile.am	(revision a554e5f423752296e46ab9ced5a34d32676e55d5)
+++ tests/Makefile.am	(revision 850aff17345d3e7433f854ef3f1b65369f8016d8)
@@ -43,4 +43,5 @@
 	-g \
 	-Wall \
+	-Werror=return-type \
 	-Wno-unused-function \
 	-Wno-psabi \
Index: tools/Makefile.am
===================================================================
--- tools/Makefile.am	(revision a554e5f423752296e46ab9ced5a34d32676e55d5)
+++ tools/Makefile.am	(revision 850aff17345d3e7433f854ef3f1b65369f8016d8)
@@ -19,5 +19,5 @@
 
 noinst_PROGRAMS = busy catchsig repeat watchdog
-AM_CFLAGS = -Wall -Wextra -O2 -g
+AM_CFLAGS = -Wall -Wextra -Werror=return-type -O2 -g
 busy_LDFLAGS     = -pthread
 
Index: tools/prettyprinter/Makefile.am
===================================================================
--- tools/prettyprinter/Makefile.am	(revision a554e5f423752296e46ab9ced5a34d32676e55d5)
+++ tools/prettyprinter/Makefile.am	(revision 850aff17345d3e7433f854ef3f1b65369f8016d8)
@@ -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