diff --git a/CMakeLists.txt b/CMakeLists.txt
index f019ab68f31..ec6c05faedb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -40,7 +40,7 @@ add_custom_target(check-style
     USES_TERMINAL
 )
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -std=c++2a -fdiagnostics-color=always")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -Wmissing-declarations -std=c++2a -fdiagnostics-color=always")
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fconcepts")
 elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")