diff --git a/.gitignore b/.gitignore
index 869d7e07d09d..7186c9d41758 100644
--- a/.gitignore
+++ b/.gitignore
@@ -235,6 +235,7 @@ META
 /parsing/camlinternalMenhirLib.ml
 /parsing/camlinternalMenhirLib.mli
 
+/runtime/api-testing
 /runtime/domain_state.inc
 /runtime/caml/m.h
 /runtime/caml/s.h
diff --git a/Makefile b/Makefile
index 5a6987e6a2b9..bc697c0cb9e5 100644
--- a/Makefile
+++ b/Makefile
@@ -1364,6 +1364,80 @@ runtime-all: \
   $(runtime_BYTECODE_SHARED_LIBRARIES:%=runtime/lib%_shared$(EXT_DLL)) \
   $(runtime_PROGRAMS:%=runtime/%$(EXE)) $(SAK)
 
+# If the compiler is configured with --enable-warn-error (which is the default
+# for development builds), then all the installed header files are tested with
+# more warnings enabled. The test also ensures that each header can be included
+# without any other headers.
+ifneq "$(EXTRA_WARNINGS_CFLAGS)" ""
+runtime-all: runtime-header-tests
+endif
+
+# As with the runtime-header-tests, if a C++ compiler is available, then we also
+# verify that the headers can be included in a C++ program without error.
+ifneq "$(EXTRA_WARNINGS_CXXFLAGS)" ""
+ifneq "$(TEST_CXX)" ""
+runtime-all: runtime-header-cxx-tests
+endif
+endif
+
+# Unlike in COMPILE_C_FILE, this is just a convenience test, so we don't do any
+# effort with the dependency computations worrying about DEP_CC and so forth,
+# because most _development_ is done with feature-rich C compilers!
+API_TESTING_DEP_FILES := $(wildcard runtime/api-testing/*.$(D))
+.PHONY: $(API_TESTING_DEP_FILES)
+include $(API_TESTING_DEP_FILES)
+
+ALL_PUBLIC_HEADER_FILES := $(notdir $(sort $(wildcard \
+  $(addsuffix /caml/*.h, $(addprefix otherlibs/, $(OTHERLIBS))) \
+  runtime/caml/*.h \
+  $(filter runtime/caml/%, $(runtime_BUILT_HEADERS)))))
+
+runtime-header-tests: \
+  $(addprefix runtime/api-testing/,$(ALL_PUBLIC_HEADER_FILES:.h=.t))
+
+runtime-header-cxx-tests: \
+  $(addprefix runtime/api-testing/,$(ALL_PUBLIC_HEADER_FILES:.h=.tpp))
+
+.PRECIOUS: runtime/api-testing
+runtime/api-testing:
+	$(MKDIR) $@
+
+runtime/api-testing/%.t: runtime/api-testing/%.c.$(O)
+	@touch $@
+
+runtime/api-testing/%.tpp: runtime/api-testing/%.cpp.$(O)
+	@touch $@
+
+runtime/api-testing/%.c.$(O): runtime/api-testing/%.c
+	$(V_CC)$(CC) \
+	  $(OC_CFLAGS) $(EXTRA_WARNINGS_CFLAGS) $(CFLAGS) \
+	  $(OC_CPPFLAGS) \
+	  $(call DEP_FLAGS,$@,$(@:.$(O)=.$(D))) \
+	  $(addprefix -I otherlibs/, $(OTHERLIBS)) $(CPPFLAGS) \
+	  $(OUTPUTOBJ)$@ -c $<
+
+runtime/api-testing/%.c: | runtime/api-testing
+	$(V_GEN){ \
+	  echo '#include <caml/$*.h>'; \
+	  echo '#include <caml/$*.h>'; \
+	  echo 'int answer = 42;'; \
+	} > $@
+
+runtime/api-testing/%.cpp.$(O): runtime/api-testing/%.cpp
+	$(V_CXX)$(TEST_CXX) \
+	  $(TEST_CXX_CXXFLAGS) $(EXTRA_WARNINGS_CXXFLAGS) $(CXXFLAGS) \
+	  $(TEST_CXX_CPPFLAGS) \
+	  $(call DEP_FLAGS,$@,$(@:.$(O)=.$(D))) \
+	  -I runtime $(addprefix -I otherlibs/, $(OTHERLIBS)) $(CPPFLAGS) \
+	  $(OUTPUTOBJ)$@ -c $<
+
+runtime/api-testing/%.cpp: | runtime/api-testing
+	$(V_GEN){ \
+	  echo '#include <caml/$*.h>'; \
+	  echo '#include <caml/$*.h>'; \
+	  echo 'int answer = 42;'; \
+	} > $@
+
 .PHONY: runtime-allopt
 ifeq "$(NATIVE_COMPILER)" "true"
 runtime-allopt: \
@@ -2709,6 +2783,7 @@ endif
 	  otherlibs/dynlink/dynlink_platform_intf.mli
 	$(MAKE) -C otherlibs distclean
 	rm -f $(runtime_CONFIGURED_HEADERS) runtime/ld.conf
+	rm -rf runtime/api-testing
 	$(MAKE) -C stdlib distclean
 	$(MAKE) -C testsuite distclean
 	rm -f tools/eventlog_metadata tools/*.bak
diff --git a/Makefile.build_config.in b/Makefile.build_config.in
index 6d3f6803163b..e2fb0309e076 100644
--- a/Makefile.build_config.in
+++ b/Makefile.build_config.in
@@ -81,6 +81,11 @@ DEP_FLAGS =
 endif
 COMPUTE_DEPS=@compute_deps@
 
+TEST_CXX = @ocamltest_CXX@
+TEST_CXX_CXXFLAGS = @common_cflags@
+TEST_CXX_CPPFLAGS = @common_cppflags@
+CXXFLAGS ?=
+
 BUILD_PATH_LOGICAL = @srcdir_abs@
 BUILD_PATH_PHYSICAL = @srcdir_abs_real@
 BUILD_MAP_FLAGS = @build_map_flags@
@@ -107,6 +112,9 @@ OC_BYTECODE_CPPFLAGS = $(OC_INCLUDES) @oc_bytecode_cppflags@
 
 OC_NATIVE_CPPFLAGS = $(OC_INCLUDES) @oc_native_cppflags@
 
+EXTRA_WARNINGS_CFLAGS = @cc_extra_warnings@
+EXTRA_WARNINGS_CXXFLAGS = @cxx_extra_warnings@
+
 # Additional link-time options
 # To support dynamic loading of shared libraries (they need to look at
 # our own symbols):
diff --git a/Makefile.common b/Makefile.common
index 23d22e015acb..a66fbcbda22e 100644
--- a/Makefile.common
+++ b/Makefile.common
@@ -46,6 +46,7 @@ V ?= 0
 ifeq "$(V)" "0"
 
 V_CC         = @$(info $   CC $@)
+V_CXX        = @$(info $   CXX $@)
 V_CCDEPS     = @$(info $   CCDEPS $@)
 V_OCAMLC     = @$(info $   OCAMLC $@)
 V_OCAMLOPT   = @$(info $   OCAMLOPT $@)
@@ -66,6 +67,7 @@ V_ODOC       = @$(info $   ODOC $@)
 else
 
 V_CC         =
+V_CXX        =
 V_CCDEPS     =
 V_OCAMLC     =
 V_OCAMLOPT   =
diff --git a/aclocal.m4 b/aclocal.m4
index 9b95b0971eb1..a1db7c2cc42d 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -651,3 +651,27 @@ AC_DEFUN([OCAML_CHECK_WINDOWS_TRIPLET], [
     [*-pc-windows*],
       [AC_MSG_ERROR([unknown MSVC variant])])
 ])
+
+# It's difficult to use AC_PROG_CXX or AX_CXX_COMPILE_STDCXX conditionally.
+# This macro is only used for ocamltest to call the C++11 compiler if the
+# default C compiler also can build C++.
+AC_DEFUN([OCAML_CXX_COMPILE_STDCXX_11], [
+  AC_CACHE_CHECK([for a C++11 compiler],
+    [ocaml_cv_prog_cxx], [
+    AS_CASE(["$ccomp_type"],
+      [cc], [
+        saved_CC="$CC"
+        CC="$CC -xc++"
+        AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+#if !defined(__cplusplus) || __cplusplus < 201103L
+#error "No C++11 support"
+#endif
+          ]])],
+          [ocaml_cv_prog_cxx="$CC"],
+          [ocaml_cv_prog_cxx=""])
+        CC="$saved_CC"],
+      [msvc], [
+        # cl.exe selects between C and C++ based on the file extension
+        ocaml_cv_prog_cxx="$CC"],
+      [ocaml_cv_prog_cxx=""])])
+  ocamltest_CXX="$ocaml_cv_prog_cxx"])
diff --git a/configure b/configure
index 031a97718565..95808a019ac7 100755
--- a/configure
+++ b/configure
@@ -801,6 +801,8 @@ build_os
 build_vendor
 build_cpu
 build
+cxx_extra_warnings
+cc_extra_warnings
 runtime_search_target
 runtime_search
 suffixing
@@ -870,6 +872,7 @@ cc_has_debug_prefix_map
 unix_directory
 unix_library
 ocamltest_libunix
+ocamltest_CXX
 ocamltest_CPP
 optional_native_tools
 optional_bytecode_tools
@@ -3568,6 +3571,7 @@ LINEAR_MAGIC_NUMBER=Caml1999L037
 
 
 
+
 
 
  # TODO: rename this variable
@@ -3631,6 +3635,8 @@ LINEAR_MAGIC_NUMBER=Caml1999L037
 
 
 
+
+
 
 
 
@@ -15426,10 +15432,14 @@ fi
 case $ocaml_cc_vendor in #(
   xlc-*) :
     warn_error_flag=''
+    cc_extra_warnings=''
+    cxx_extra_warnings=''
     cc_warnings='-qflag=i:i' ;; #(
   # all warnings enabled
   sunc-*) :
-    cc_warnings="" ;; #(
+    cc_extra_warnings=''
+     cxx_extra_warnings=''
+     cc_warnings='' ;; #(
   msvc-*) :
     case $ocaml_cc_vendor in #(
   msvc-*-clang-*) :
@@ -15437,10 +15447,14 @@ case $ocaml_cc_vendor in #(
          warn_error_flag='-WX' ;; #(
   *) :
     cc_warnings='-W2'
-       warn_error_flag='-WX -options:strict' ;;
+       cc_extra_warnings='-permissive-'
+       warn_error_flag='-WX -options:strict'
+       cxx_extra_warnings="$cc_extra_warnings $warn_error_flag" ;;
 esac ;; #(
   *) :
     warn_error_flag='-Werror'
+  cc_extra_warnings='-Wextra -Wpedantic -pedantic -std=c11'
+  cxx_extra_warnings='-Werror -Wextra -Wpedantic -pedantic'
   cc_warnings="-Wall -Wint-conversion -Wstrict-prototypes \
 -Wold-style-definition -Wundef" ;;
 esac
@@ -15581,7 +15595,8 @@ case $enable_warn_error,true in #(
   yes,*|,true) :
     cc_warnings="$cc_warnings $warn_error_flag" ;; #(
   *) :
-     ;;
+    cc_extra_warnings=''
+    cxx_extra_warnings='' ;;
 esac
 
 case $target in #(
@@ -23881,7 +23896,54 @@ else $as_nop
 testsuite/tools/test_in_prefix"
 fi
     testsuite_tools='testsuite/tools/codegen testsuite/tools/expect'
-    optional_bytecode_tools="$optional_bytecode_tools $testsuite_tools" ;; #(
+    optional_bytecode_tools="$optional_bytecode_tools $testsuite_tools"
+
+  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for a C++11 compiler" >&5
+printf %s "checking for a C++11 compiler... " >&6; }
+if test ${ocaml_cv_prog_cxx+y}
+then :
+  printf %s "(cached) " >&6
+else $as_nop
+
+    case "$ccomp_type" in #(
+  cc) :
+
+        saved_CC="$CC"
+        CC="$CC -xc++"
+        cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+#if !defined(__cplusplus) || __cplusplus < 201103L
+#error "No C++11 support"
+#endif
+
+int
+main (void)
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"
+then :
+  ocaml_cv_prog_cxx="$CC"
+else $as_nop
+  ocaml_cv_prog_cxx=""
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.beam conftest.$ac_ext
+        CC="$saved_CC" ;; #(
+  msvc) :
+
+        # cl.exe selects between C and C++ based on the file extension
+        ocaml_cv_prog_cxx="$CC" ;; #(
+  *) :
+    ocaml_cv_prog_cxx="" ;;
+esac
+fi
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $ocaml_cv_prog_cxx" >&5
+printf "%s\n" "$ocaml_cv_prog_cxx" >&6; }
+  ocamltest_CXX="$ocaml_cv_prog_cxx" ;; #(
   *) :
     build_ocamltest=false
   ocamltest='' ;;
diff --git a/configure.ac b/configure.ac
index 3d918e242f89..b0e3953797be 100644
--- a/configure.ac
+++ b/configure.ac
@@ -219,6 +219,7 @@ AC_SUBST([optional_libraries])
 AC_SUBST([optional_bytecode_tools])
 AC_SUBST([optional_native_tools])
 AC_SUBST([ocamltest_CPP])
+AC_SUBST([ocamltest_CXX])
 AC_SUBST([ocamltest_libunix])
 AC_SUBST([unix_library])
 AC_SUBST([unix_directory])
@@ -288,6 +289,8 @@ AC_SUBST([native_runtime_id])
 AC_SUBST([suffixing])
 AC_SUBST([runtime_search])
 AC_SUBST([runtime_search_target])
+AC_SUBST([cc_extra_warnings])
+AC_SUBST([cxx_extra_warnings])
 
 ## Generated files
 
@@ -1035,17 +1038,25 @@ AS_IF([test "x$interpval" = "xyes"],
 AS_CASE([$ocaml_cc_vendor],
   [xlc-*],
     [warn_error_flag=''
+    cc_extra_warnings=''
+    cxx_extra_warnings=''
     cc_warnings='-qflag=i:i'], # all warnings enabled
   [sunc-*],
-    [cc_warnings=""],
+    [cc_extra_warnings=''
+     cxx_extra_warnings=''
+     cc_warnings=''],
   [msvc-*],
     [AS_CASE([$ocaml_cc_vendor],
       [msvc-*-clang-*],
         [cc_warnings='-W4 -Wno-unused-parameter -Wno-sign-compare -Wundef'
          warn_error_flag='-WX'],
       [cc_warnings='-W2'
-       warn_error_flag='-WX -options:strict'])],
+       cc_extra_warnings='-permissive-'
+       warn_error_flag='-WX -options:strict'
+       cxx_extra_warnings="$cc_extra_warnings $warn_error_flag"])],
   [warn_error_flag='-Werror'
+  cc_extra_warnings='-Wextra -Wpedantic -pedantic -std=c11'
+  cxx_extra_warnings='-Werror -Wextra -Wpedantic -pedantic'
   cc_warnings="-Wall -Wint-conversion -Wstrict-prototypes \
 -Wold-style-definition -Wundef"])
 
@@ -1068,7 +1079,9 @@ done
 
 AS_CASE([$enable_warn_error,OCAML__DEVELOPMENT_VERSION],
   [yes,*|,true],
-    [cc_warnings="$cc_warnings $warn_error_flag"])
+    [cc_warnings="$cc_warnings $warn_error_flag"],
+    [cc_extra_warnings=''
+    cxx_extra_warnings=''])
 
 AS_CASE([$target],
   [*-w64-mingw32*|*-pc-windows],
@@ -2831,7 +2844,8 @@ testsuite/tools/test_in_prefix.opt"],
       [optional_bytecode_tools="$optional_bytecode_tools \
 testsuite/tools/test_in_prefix"])
     testsuite_tools='testsuite/tools/codegen testsuite/tools/expect'
-    optional_bytecode_tools="$optional_bytecode_tools $testsuite_tools"],
+    optional_bytecode_tools="$optional_bytecode_tools $testsuite_tools"
+    OCAML_CXX_COMPILE_STDCXX_11],
   [build_ocamltest=false
   ocamltest=''])
 
diff --git a/runtime/caml/sizeclasses.h b/runtime/caml/sizeclasses.h
index ab4e7e37ffdd..86a1af361508 100644
--- a/runtime/caml/sizeclasses.h
+++ b/runtime/caml/sizeclasses.h
@@ -1,4 +1,8 @@
 /* This file is generated by tools/gen_sizeclasses.ml */
+
+#ifndef CAML_SIZECLASSES_H
+#define CAML_SIZECLASSES_H
+
 #define POOL_WSIZE 4096
 #define POOL_HEADER_WSIZE 4
 #define SIZECLASS_MAX 128
@@ -59,3 +63,5 @@ static const sizeclass sizeclass_wsize[SIZECLASS_MAX + 1] =
   /*115:*/ 30, 30, 30, 30, 31,
   /*120:*/ 31, 31, 31, 31, 31,
   /*125:*/ 31, 31, 31, 31 };
+
+#endif /* CAML_SIZECLASSES_H */
diff --git a/tools/gen_sizeclasses.ml b/tools/gen_sizeclasses.ml
index 513539e808a0..40093893f164 100644
--- a/tools/gen_sizeclasses.ml
+++ b/tools/gen_sizeclasses.ml
@@ -89,6 +89,8 @@ let print_list ppf li =
 
 let _ =
   printf "/* This file is generated by tools/gen_sizeclasses.ml */\n";
+  printf "\n#ifndef CAML_SIZECLASSES_H\n";
+  printf "#define CAML_SIZECLASSES_H\n\n";
   printf "#define POOL_WSIZE %d\n" arena;
   printf "#define POOL_HEADER_WSIZE %d\n" header_size;
   printf "#define SIZECLASS_MAX %d\n" max_slot;
@@ -115,3 +117,4 @@ static const unsigned char wastage_sizeclass[NUM_SIZECLASSES] =@[<2>{ %a };@]
 static const sizeclass sizeclass_wsize[SIZECLASS_MAX + 1] =@[<2>{ %a };@]
 |}
     print_list (255 :: size_slots 1);
+  printf "\n#endif /* CAML_SIZECLASSES_H */\n"