From: Valentin Bonnard <Bonnard.V@[...]..> 
>  ...
>  /users/bonnard/debug_mode.hpp:
>  > extern const bool debug_mode = true
> 
>  /users/bonnard/debug_mode.cpp:
>  > #include debug_mode.hpp
>  > const bool debug_mode;

I think that, despite the extern, the declaration in debug_mode.hpp is a
definition, because of the initializer:


3.1 Declarations and definitions [basic.def]

1 A declaration (clause _dcl.dcl_) introduces names into a translation
unit or redeclares names introduced by previous declarations. A dec-
laration specifies the interpretation and attributes of these names.

2 A declaration is a definition unless it declares a function without
specifying the function's body (_dcl.fct.def_), it contains the extern
specifier (_dcl.stc_) or a linkage-specification1) (_dcl.link_) and
neither an initializer nor a function-body, it declares a static data
member in a class declaration (_class.static_), it is a class name
declaration (_class.name_), or it is a typedef declaration (_dcl.type-
def_), a using-declaration (_namespace.udecl_), or a using-directive
(_namespace.udir_).