Enforce not shadowing variables

Shadowing variable can easily lead to bugs, so let's enforce that this
is not allowed.

CMakeLists.txt

@@ -79,6 +79,7 @@ set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Wformat=2 -Wvla")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wformat=2 -Wvla")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wzero-as-null-pointer-constant")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wsuggest-override")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wshadow")
 # Make sure we catch these issues whilst developing
 IF(CMAKE_BUILD_TYPE MATCHES Debug)
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")