diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 68d69eded3b6cf4a1ed88cee1c040b9c8b30de6d..69953665265a7fa0772236b8a178e17fabb0b9d8 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -144,10 +144,10 @@ set(api_headers
     ${CMAKE_SOURCE_DIR}/src/lua/error.h
     ${CMAKE_SOURCE_DIR}/src/lua/string.h
     ${CMAKE_SOURCE_DIR}/src/box/txn.h
+    ${CMAKE_SOURCE_DIR}/src/box/tuple.h
     ${CMAKE_SOURCE_DIR}/src/box/key_def.h
     ${CMAKE_SOURCE_DIR}/src/box/lua/key_def.h
     ${CMAKE_SOURCE_DIR}/src/box/field_def.h
-    ${CMAKE_SOURCE_DIR}/src/box/tuple.h
     ${CMAKE_SOURCE_DIR}/src/box/tuple_format.h
     ${CMAKE_SOURCE_DIR}/src/box/tuple_extract_key.h
     ${CMAKE_SOURCE_DIR}/src/box/schema_def.h
diff --git a/src/box/index.h b/src/box/index.h
index 86148023f1223dcde53bde85b49a5f99b6b165f0..6225a8674758dd50e0979c7446ec40d2a6b8f70b 100644
--- a/src/box/index.h
+++ b/src/box/index.h
@@ -46,10 +46,11 @@ struct index_def;
 struct key_def;
 struct info_handler;
 
-/** \cond public */
-
 typedef struct tuple box_tuple_t;
 typedef struct key_def box_key_def_t;
+
+/** \cond public */
+
 typedef struct iterator box_iterator_t;
 
 /**
diff --git a/src/box/key_def.h b/src/box/key_def.h
index 9f358687a2ce5dd82eca7337b634134352edf6a0..0f9b4a4a8247e7e0f423130b3b59111f7aa0f74d 100644
--- a/src/box/key_def.h
+++ b/src/box/key_def.h
@@ -287,10 +287,11 @@ key_def_copy(struct key_def *dest, const struct key_def *src);
 void
 key_def_delete(struct key_def *def);
 
+typedef struct tuple box_tuple_t;
+
 /** \cond public */
 
 typedef struct key_def box_key_def_t;
-typedef struct tuple box_tuple_t;
 
 /**
  * Create key definition with key fields with passed typed on passed positions.
diff --git a/test/app-tap/CMakeLists.txt b/test/app-tap/CMakeLists.txt
index bf7d28136b41e44846e52adb1c67415e66780151..0fb34711ecde496a942b8a80aa4cc51874615e66 100644
--- a/test/app-tap/CMakeLists.txt
+++ b/test/app-tap/CMakeLists.txt
@@ -1,2 +1,8 @@
 build_module(module_api module_api.c)
 build_module(libyield libyield.c)
+
+# gh-5313: verify that module.h actually conforms to the C99
+# standard.
+set(CMAKE_C_FLAGS "-Wall -Wextra -std=c99")
+set(CMAKE_C_FLAGS_DEBUG "-g -O0 -Werror")
+set(CMAKE_C_FLAGS_RELWITHDEBINFO "-O2")
diff --git a/test/app-tap/module_api.c b/test/app-tap/module_api.c
index b81a98056575a99060eab1db6623ba23f249a51d..a79fbed0d5aa5f566c51cad33369e099774b98fa 100644
--- a/test/app-tap/module_api.c
+++ b/test/app-tap/module_api.c
@@ -198,6 +198,7 @@ test_toint64(lua_State *L)
 
 int fiber_test_func(va_list va)
 {
+	(void) va;
 	do {
 		fiber_set_cancellable(true);
 		fiber_sleep(0.01);
@@ -380,6 +381,7 @@ test_call(lua_State *L)
 static int
 cpcall_handler(lua_State *L)
 {
+	(void) L;
 	return 0;
 }